sixtracklib/common: add missing accessor functions to NS(Buffer) API
sixtracklib/common: add missing accessor functions to NS(Buffer) API

- Provide NS(Buffer_get_const_object) and NS(Buffer_get_object)
  Accessor functions for directly accesing the i-th NS(Object) stored into the
  buffer without having to rely on the iterator interface

  this had been used in the C++ bindings operator[] but not provided yet ->
  fix the resulting problems!

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