common: fixes linking problems arising from common/particles.hpp
common: fixes linking problems arising from common/particles.hpp

- Three methods for the specialized TParticles< double > variant
  had been declared static but not been inlined properly, causing
  ODR violations when linking the shared library. This is fixed now.

- Sanitizes the header to not cause problems if it is (accidentially)
  included in a C99 context by properly hiding most of the C++ part
  behind preprocessor guards

- Some cosmetic changes concerning the preprocessor macros

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib