add README files for the DummyUsrFcn with an example.

doc/examples/UserFcnWithGlobal/README

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+
+Simple Example for a User Function *with* a Global Part
+
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+
+The source code is found under 
+
+<musrfit-dir>src/external/DummyUserFcn
+
+For details see there.
+
+This dummy user function implements an exp(-lambda*t)
+in the global part. This is the most efficient way, 
+since the function time vector is only calculated once
+when the parameter (lambda) changes. For all the 
+subsequent calls a lookup table is used to return the
+value of the user function.

doc/examples/UserFcnWithGlobal/test-histo-PSI-BIN-UserFcn.msr

+FeSe 9p4 TF100 p107apr09_sample*1p02                          
+###############################################################
+FITPARAMETER
+#      Nr. Name       Value     Step      Pos_Error Boundaries   
+        1 Asy         0.2622    -0.0014     0.0014      0       0.33    
+        2 Rate        0.3188    -0.0044     0.0044      
+        3 Field       97.853    -0.056      0.056       0       200     
+        4 Phase_L     178.95    -0.41       0.41        
+        5 Phase_R     1.75      -0.39       0.39        
+        6 N0_L        1097.9    -1.0        1.0         
+        7 N0_R        1159.7    -1.0        1.0         
+        8 Bkg_L       54.47     -0.20       0.20        
+        9 Bkg_R       46.70     -0.19       0.19        
+
+###############################################################
+THEORY
+asymmetry      1
+userFcn libPDummyUserFcn   PDummyUserFcn    2          (rate)
+TFieldCos   map1  fun1       (phase frequency)
+
+###############################################################
+FUNCTIONS
+fun1 = par3 * gamma_mu
+
+###############################################################
+GLOBAL
+fittype         0         (single histogram fit)
+fit             0       8.2    
+packing         1
+
+###############################################################
+RUN ../data/deltat_pta_gpd_0423 PIE1 PSI PSI-BIN   (name beamline institute data-file-format)
+norm            6
+backgr.fit      8
+map             4    0    0    0    0    0    0    0    0    0
+forward         1 
+data            165     7965    
+t0              162.0   
+
+RUN ../data/deltat_pta_gpd_0423 PIE1 PSI PSI-BIN   (name beamline institute data-file-format)
+norm            7
+backgr.fit      9
+map             5    0    0    0    0    0    0    0    0    0
+forward         2 
+data            205     7965    
+t0              202.0   
+
+###############################################################
+COMMANDS
+SCALE_N0_BKG TRUE
+MINIMIZE
+MINOS
+SAVE
+
+###############################################################
+PLOT 0   (single histo plot)
+lifetimecorrection
+runs     1  2
+range    0.0  9.5  -0.3 0.3
+view_packing 25
+
+###############################################################
+FOURIER
+units            Gauss   # units either 'Gauss', 'Tesla', 'MHz', or 'Mc/s'
+fourier_power    12
+apodization      NONE    # NONE, WEAK, MEDIUM, STRONG
+plot             REAL    # REAL, IMAG, REAL_AND_IMAG, POWER, PHASE
+phase            par4 par5
+range            0.0    200.0
+
+###############################################################
+STATISTIC --- 2015-01-05 14:09:47
+  chisq = 663.9, NDF = 515, chisq/NDF = 1.289169

src/external/DummyUserFcn/README

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+
+Simple Example for a User Function *with* a Global Part
+
+/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
+
+This is a full implementation of a user function with a global part.
+For a general intro to user function, please check the manual under
+
+http://lmu.web.psi.ch/musrfit/user/html/user-manual.html#id38
+
+The implemented user function is
+
+exp(-lambda*t)
+
+with lambda, being the only parameter.
+
+Short description of the PDummyUserFcn and PDummyUserFcnGlobal classes.
+
+PDummyUserFcn:
+-------------
+
+This class derives from the PUserFcnBase which is necessary to make this
+user function available to musrfit.
+
+The three functions:
+
+    virtual Bool_t NeedGlobalPart() const { return true; }
+    virtual void SetGlobalPart(std::vector<void*> &globalPart, UInt_t idx);
+    virtual Bool_t GlobalPartIsValid() const;
+
+are used to bind the global part of the user function.
+
+The method
+
+    virtual Double_t operator()(Double_t t, const std::vector<Double_t> &param) const;
+
+is called by musrfit when evaluate the function.
+
+The private variables:
+
+    Bool_t fValid{true};
+    Bool_t fInvokedGlobal{false};
+    Int_t fIdxGlobal;
+
+    PDummyUserFcnGlobal *fDummyUserFcnGlobal{nullptr};
+
+have the following purpose:
+
+    fValid is a boolean variable holding the state of the user function. 
+           If true, it means that the user function is useable. In case
+           something would go wrong during the initialization stage fValid
+           is set to false, and no calculation will take place!
+    fInvokedGlobal is a boolean variable which is set to true is the global
+           part of the user function is properly invoked.
+    fIdxGlobal holds the index of the global user function part.
+ 
+PDummyUserFcnGlobal:
+-------------------
+
+This class holds the gloabl user function part, which handles the actual 
+calculation of the user function.
+
+    CalculatePol(const std::vector<Double_t> &param) const; -> carries out the
+          necessary calculations of this user function. This is ONLY carried 
+          out if the parameter set has changed!
+
+    GetPolValue(const Double_t t) const; -> This is used in the operator() of
+          the user function to get the value of the user function (here a 
+          value of a polarization function exp()).
+
+    mutable std::vector<Double_t> fPreviousParam; -> holds the previous 
+          parmeter set in order to check if a parameter set has changed.
+
+    mutable std::vector<Double_t> fPol; -> lookup table for the polarization
+          function for the current parameter set.
+
+How to build:
+------------
+
+When configure musrfit, there is a switch DummyUserFcn available which
+needs to be enabled (default: OFF), e.g.
+
+cmake ../ -DCMAKE_INSTALL_PREFIX=$ROOTSYS -Dnexus=1 -DDummyUserFcn=1
+
+Files:
+-----
+  CMakeLists.txt <- this File is needed to allow to build an install
+                    this dummy example with the cmake build system.
+  inc/PDummyUserFcn.h  <- header file definig the needed classes for
+                          the user function and its globale part. 
+  inc/PDummyUserFcnLinkDef.h <- Needed as a glue to generate the necessary
+                                root dictionaries.
+  src/CMakeLists.txt <- needed for the cmake build/install process.
+  src/PDummyUserFcn.cpp <- source code which implements all the necessary
+                           parts of the user function.
+  src/PDummyUserFcn.pc.in <- cmake input file to generate the propoer 
+                             pkg-config file for the library
+