minor correction for calculating the ref 4 vec system of the mother ref system; needed for helicity formalism

Started to port Jamroot etc. to be used also with CLANG80/LLVM as an alternative compiler. Also fixed some minor issues according to C++ standard conformity that G++ usually ignores (due to 'language extensions').

pretection against round-off errors of 4-vectors with masses around 0 (e.g. photons) in dyn functions