combine/optimize _match_vertices and find_point_permutation implementations (#458, part 2)

* optimize vertex matching in periodicity processing

* use np.median instead of approximating with np.histogram

    turns out to be just as fast

* combine _match_vertices and mesh.tools.find_point_permutation into find_point_to_point_mapping

* add tests for find_point_to_point_mapping

* use find_point_to_point_mapping instead of find_point_permutation