Create named subclasses of MeshFunction

It is somewhat difficult/confusing to create MeshFunctions since one must specify the topological dimension.

We could subclass MeshFunction to create named MeshFunctions of specific dimensions. Instead of

MeshFunction<uint> f(mesh, mesh.topology().dim())

we could have

CellFunction<uint> f(mesh);

The list of named MeshFunctions would be

VertexFunction
EdgeFunction
FaceFunction
FacetFunction
CellFunction

This would also be consistent with the named MeshEntity classes and iterators (but slightly inconsistent for the naming of MeshFunction which should really be MeshEntityFunction).