> Why mat_mesh?

The function name is in the likeness of the existing ```flux_mesh_to_fluxin```.

> don't all meshes have materials?

Not necessarily. You can instantiate a ```Mesh``` object in the ```Meshtally``` or ```Wwinp``` class without materials. In many workflows you might not have the means of knowing, and might not even care, about the material composition in each volume element of a superimposed mesh. This is certainly true in weight window generation.

 By ```mat_mesh``` I am trying to say "the ```Mesh``` object in your workflow that has all the material information on it." Note that depending on what is being done, it might be impractical to store all data on a single ```Mesh``` object.

Nonetheless, I am happy to change it if you still disagree.