clang compiler fix

This MR fixes compilation with clang of the master branch.

In the c++11 standard sqrt and pow are not necessarily constexpr functions (e.g. https://stackoverflow.com/questions/8622256/in-c11-is-sqrt-defined-as-constexpr). With the clang compiler I get the following errors:

OPAL/fork/src/src/Classic/Solvers/CollimatorPhysics.cpp:331:22: error: 
      constexpr variable 'K' must be initialized by a constant expression
  ...double K = 4.0 * pi * Physics::Avo * std::pow(Physics::r_e * m2cm, 2) * massElectron_keV;
            ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/jsnuverink/Documents/OPAL/fork/src/src/Classic/Solvers/CollimatorPhysics.cpp:331:52: note: 
      non-constexpr function 'pow<double, int>' cannot be used in a constant expression
    constexpr double K = 4.0 * pi * Physics::Avo * std::pow(Physics::r_e * m2cm, 2) * mass...


OPAL/fork/src/src/Classic/Solvers/CollimatorPhysics.cpp:429:22: error: 
      constexpr variable 'sqrtThreeInv' must be initialized by a constant expression
    constexpr double sqrtThreeInv = 1.0 / std::sqrt(3.0);
                     ^              ~~~~~~~~~~~~~~~~~~~~
/Users/jsnuverink/Documents/OPAL/fork/src/src/Classic/Solvers/CollimatorPhysics.cpp:429:43: note: 
      non-constexpr function 'sqrt' cannot be used in a constant expression
    constexpr double sqrtThreeInv = 1.0 / std::sqrt(3.0);
                                          ^
/usr/include/math.h:447:15: note: declared here
extern double sqrt(double);
              ^

Compiler version:

Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

In addition I deleted the using Physics::pi to improve consistency, since the Physics namespace was everywhere apart from one location explicitly used.