Complete cell integral generation for ffc

[Martin Sandve Alnæs]

To make the implementation of cell integrals feature complete, uflacs needs to implement: - Proper selection of quadrature rule (currently hardcoded choice) - Geometry quantities such as cell volume etc - Non-affine vector element mappings - Non-affine derivative mappings - Higher order derivatives - Quadrature elements - Multiple integration rules - Anything else?

Optimizations are not considered in this blueprint.