Create mesh from face-vertex (and more)

* mesh from face-vertex, closed meshes only for now
* add PlanktonHalfEdgeList containing storage and functions for half-edges (see comments for more info)
* store pairs in half-edges as an alternative to storing half-edges in order
* dual updated to use stored pairs
Improve performance of pair-matching function

* using a Hashtable
* keys use Szudzik's pairing function