Define concept of FunctionSpace for mixed topology

[chrisrichardson]

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.