Separate face matching from _compute_facial_adjacency_from_vertices (#207)

* refactor _compute_facial_adjacency_from_vertices so that face matching can be called separately

* document array shapes in _FaceIDs

* document _match_faces_by_vertices

* change "matches" to "face_index_pairs"

* simplify some array initializations

* temporarily restore _FlatFacialAdjacencyData

* fix docstrings

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* add comment explaining initial sorting of vertex indices

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* reword comment

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* improve documentation of _match_faces_by_vertices

* Revert "temporarily restore _FlatFacialAdjacencyData"

This reverts commit fe50f623b4082aa12d1ef05fd545e688ed1ba507.

Co-authored-by: Andreas Klöckner <inform@tiker.net>