Define concept of `FunctionSpace` for mixed topology

When a mesh contains multiple cell types (e.g. triangle + quadrilateral), we need to be able to define a `FunctionSpace` on that mesh, so we can add `Forms`.
Currently, a `FunctionSpace` encapsulates: `[Mesh, FiniteElement, DofMap]`. I suggest we could change it to: `[Mesh, [FiniteElements], [DofMaps]]` where `[FiniteElements]` and `[DofMaps]` are lists which must be the same size as the number of cell types in the mesh.