Add hash value for mesh to ufc::cell

The signature for ufc::function::evaluate is

void evaluate(double* values,
                      const double* coordinates,
                      const ufc::cell& cell) const

Sub classes must return the value based on the given coordinates and the cell.

The problem is that (in for example interpolation to different meshes) the cell might originate from another mesh and sub classes have no way to know whether the cell belongs to a different mesh.

We could fix this by adding a hash value for the mesh to ufc::cell:

unsigned int mesh_hash;

This would require the addition of a signature() and a hash() function in dolfin::Mesh.