- allow to shift a particle distribution (needed for multibunch)
- generalize the phyiscal domain creation, each dimension can have
  a different extent (needed for multibunch)

modified:   Distribution.cpp
modified:   Distribution.h
modified:   helper_functions.h
modified:   testGaussian.cpp
modified:   testGridSolve.cpp
modified:   testReal.cpp
modified:   testUnifSphere.cpp
modified:   testUnifSphereGrid.cpp