Rename mesh_material to mesh_subgroup

[Foxar]

I feel like mesh_material is an incorrect name for the class considering that it is responsible for mesh data of sub-parts of another, bigger mesh.

[Foxar]

@h4570 What do you think about this issue, is it worth changing or should we keep the name as is?

[h4570]

Resolved in Tyra 2.0.0