sixtracklib/common: Added beam elements API header
sixtracklib/common: Added beam elements API header

The header file impl/beam_elements_api.h is expected to be generated eventually automatically (i.e. by introspection).

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