Changing the representation of spatial derivatives of functions to match new ufl differentiation algorithm

A new ufl differentiation algorithm exists, which produces gradients of functions represented as Grad(Coefficient()) instead of SpatialDerivative(Coefficient(), MultiIndex()). FFC needs to be able to handle this representation before the new algorithm can be made default in UFL.

The main reason for this change is that it makes a unique representation of gradients, as opposed to the index notation where v.dx(i) != v.dx(j). This is important for some new form compiler algorithms that I'm working on which scales much better to complex expressions.