sixtracklib/common: adds missing handling of m_num_stored_buffers during construction and assignment
sixtracklib/common: adds missing handling of m_num_stored_buffers during construction and assignment

NOTE: apparently, m_stored_buffers and m_assign_item_keys are not handled during copy-assignment
      operations -> this is not good and should be fixed!

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