Add edge split and prevent non-manifold edge in edge flip
Add face split and more

* New SplitFace method
* New He_AroundFace method
* Rename MakeAdjacent to MakeConsecutive
Added Collapse/Remove methods. Other cleanup

Renamed He_AroundFace to GetNextHalfEdge
Moved SplitFace method to FaceList
Added RemoveEdge method
Added CollapseEdge method
Added 'Dead' bools to vertices, faces and halfedges
Check for dead items when converting
fix in SplitEdge

Forgot to set the new vertex as the start of the new edge
Fixed a bug in SplitEdge
Added TriangleSplitEdge

Combined split edge & 2 split faces to split 2 triangles into 4
Add unit test for edge split
Fix bug in face split and add test

* re-order operations within face split to fix bug where halfedges were incorrectly linked, preventing proper traversal
* stream-line using existing face-halfedge circulator to link halfedges to face
* add a unit test for face split method
Modify face split to deal exclusively in halfedges

* accepts two halfedge indices as arguments, new edge between the start vertices
* returns the index one of the newly added halfedges (from->next infact)
* test updated
Move RemoveEdge to MergeFaces and simplify EdgeCollapse

* MergeFaces is now reverse of SplitFace
* chained together they leave mesh unchanged
* more checks in merge (no boundary- or multi-edges)
* EdgeCollapse modifed according to CGAL's vertex join (WIP, no unit test)
Improve face split/merge unit tests
Fix edge collapse and add test coverage

* forgot to update faces' first halfedges where necessary
* unit tests for both internal and boundary edge collapses
Update edge collapse operator

* don't allow if it would create a non-manifold vertex
* merge degenerate faces (< 2 sides)
* if merge not possible, we should probably remove the face (but this isn't implemented yet..)
Add face removal to complete edge collapse (plus tests)

* add ability to remove faces (with test)
* add missing face removal to edge collapse (now handles adjacent triangular face which can't be merged)
* new edge collapse test for triangular faces
* in halfedge list, make add pair method internal (it doesn't maintain validity)
* add ability to remove pair of halfedges (not public either)
* add easy way of checking if edge is boundary
Added stellate method

added PlanktonVertex from PlanktonXYZ
fixed a typo in 1 comment
Add vertex join/split and tidy up

* add vertex split (+ tests)
* add vertex merge (= edge collapse)
* tidy up vertex list
* add simple vertex boundary check for future use
* fix typo in halfedge boundary check
Fix bug in edge split, tidy up and update test

* fix bug where outgoing halfedge was being updated for the wrong vertex
* set temporary position for new vertex (copy end vertex)
* more in-depth testing (check for working circulators at each vertex)
Added ToPoint3d methods

made EndVertex public
changed SplitEdge to use midpoint