python/sixtracklib: fixes interpol.py implementation
python/sixtracklib: fixes interpol.py implementation

- fixes type error for casting the base address of the CObjects instance
  to a CTypes pointer
- updates the constructor to handle complex scenarios with capacity/num_values
- provides a convenience function for mapping NS(LineProfileDensityData) to
  NS(SpaceChargeInterpolatedProfile) instances

TODO: Consider expanding the constructor to directly handle values and
      derivatives during the construction of an instance

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