Rename PairHalfedge to GetOpposite (and tidy)

* PairHalfedge suggests "pairing a halfedge" to me...
* PairHalfedge still usable, for now
* Organise source by moving methods to another 'region'
Fix a bug that I introduced to edge collapse

* I forgot to update the faces' first halfedge index
* reverting to merge/delete method
* add limits to halfedge circulators
* should error out before consuming all memory!
Rename GetOpposite to GetPairHalfedge

* everything updated
* PairHalfedge marked as obsolete
Remove cull parameter from ToRhinoMesh

We now have native cull/compaction