mesh_from_array() does not include normals

[stuarteberg]

The mesh_from_array() function obtains vertices, normals, and faces from the marching_cubes library:

 vertices_xyz, _normals, faces = march(volume_xyz, smoothing_rounds)

...but we don't actually write the normals into the .obj file. Apparently this creates a performance issue for NeuTu.

Someone should investigate whether or not there is a good reason we're not writing the normals. (Are the normals incorrect? Do they pose a problem for fq-mesh-simplify or draco_encoder?)

Unless there's a compelling reason not to, we should write the normals into the .obj instead of simply discarding them.