MeshGenerators aren't really generators

[jwallwork23]

"But more generally... connected to point 2, I find the name *Generator a bit misleading since I'd then expect it to generate a different mesh every time we call generate_mesh(). Why not just call it UnstructuredSquareMesh (and similarly for others) since generate_mesh() always generates the same mesh?" - originally posted by @ddundo in review of #40.