compute_normals() not in Python API

The [scenepic::Mesh::compute_normals()](https://microsoft.github.io/scenepic/cpp/classscenepic_1_1_mesh.html#a5a85f330c437d206661f8a6a66388389) function in the C++ API is not exposed in the Python [scenepic.Mesh](https://microsoft.github.io/scenepic/python/graphics3d.html#mesh) API. Is it possible to add this?