filter.cutEdges, filter.addVertexLike, convert.deepCopy, convert.edge…

…s_vertices_to_vertices_edges_sorted, convert.vertices_edges_to_faces_vertices_edges, convert.edges_vertices_to_faces_vertices_edges

* Add `FOLD.filter.cutEdges` for cutting apart ("unwelding") edges
  in 2D crease pattern, using `vertices_edges`
* `vertices_edges` manipulators:
  * Add `FOLD.convert.vertices_edges_to_faces_vertices_edges`
  * Add `FOLD.convert.edges_vertices_to_vertices_edges_sorted` helper
  * Add `FOLD.convert.edges_vertices_to_faces_vertices_edges` helper
  * Rename `FOLD.convert.edges_vertices_faces_vertices_to_faces_edges`
    to `FOLD.convert.faces_vertices_to_faces_edges`.
* Helpers:
  * Add `FOLD.filter.addVertexLike` similar to `FOLD.filter.addEdgeLike`
    (for vertex duplication)
  * Re-order arguments of `FOLD.filter.addEdgeLike` (for edge duplication)
  * Add `FOLD.convert.deepCopy` for deep copying fold structures
  * Improve `FOLD.filter.numType` (and thus `FOLD.filter.numVertices`)
    to use `..._type` keys when there are no `type_...` keys.
  * Change `filter.edges_verticesIncident` to return common vertex
    when the answer is "yes" (instead of `true`),
    and `null` otherwise (instead of `false`).
* Update documentation