sixtracklib: use sixtracklib/common/output_buffer.h in convenience header rather than io_buffer.h
sixtracklib: use sixtracklib/common/output_buffer.h in convenience header rather than io_buffer.h

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