Face modifications

[robin-oval]

Functions to modify the vertices of a face ([a, b, c, d] -> [a, b, c, e] or [a, b, c, d] -> [a, b, c, d, e] for instance) that combines delete_face and add_face or that modifies directly the face vertices while updating the halfedges.

[tomvanmele]

has this already been addressed?

[robin-oval]

I'll make a PR for this soon.