Mesh mats

[pshriwise]

I was reviewing #56 and I'm a little confused as to what the mats data structure is intended to be...

By default it looks like an empty tuple, but the documentation states it should be a MaterialLibrary or dict of mats (typo in docstring there) mapping volume element handles to materials. Here we're mapping indices to materials.

I see that the materials object always gets set to a valid dictionary-like object unless mats is None, indicating that the user doesn't want materials on the mesh.

• I think that at some point we should figure out what's up with the keys for the mats property. I'm thinking it's just a docstring update? 🤞

• I'll also advocate for the default of mats to be an empty dictionary (or MaterialLibrary) to reduce confusion about what the data structure ends up being.