New ffc-x interface for integrals

[garth-wells]

From @chrisrichardson

At the moment, ffc generates tabulate_tensor() code for integrals in several ufc::foointegral classes, for cell, interior_facet, exterior_facet, vertex etc. The class also contains an enabled_coefficients() function (can we eliminate?)

Should we use a common function signature for all integrals?
e.g.
void tabulate_tensor(double *A, const double* w, const double* coords, const int* entity_indices);

(for integrals over multiple cells: "macro", the data can be stacked for each argument).
We can store the integrals as std::function<> or just as raw C function pointers (may be more efficient).

[x] signature for cell
[x] signature for interior_facet
[x] signature for exterior_facet
[x] signature for vertex
[] signature for custom integral

[garth-wells]

Fixing this will address #33.

[chrisrichardson]

Closing, as this is 90% done. Remaining issue is for custom integrals.