Contrib TriangleSoup from_gltf() method

In our Py3DTiles, we've writen a from_gltf() method to create a TriangleSoup instance from the gltf attribute of a B3DM.

Since the method is used by Py3DTileRs Tileset Reader, this method should be either contributed to Oslandia or implemented directly in Py3DTilers (if OSlandia isn't interested)