Request standard properties on demand
The openmesh python bindings should request standard properties (i.e. vertex normals) on demand.
For example:
the wrapper for normal(vh)
should first check if vertex normals are available with mesh.has_vertex_normals()
and request them if necessary before returning the normal.
This avoids unexpected crashes, while also circumventing the unnecessary copying around of unneeded vectors if we just request all standard properties for our mesh.