Don't throw away supporting entities of dofs

[Jan Blechta]

Information about supporting entities of dofs is thrown away in some subclasses of FiniteElement, e.g. DiscontinuousLagrange. The information is readily available - support entities of DiscontinuousLagrange are the same as Lagrange.entity_dofs() - but just thrown away.

The information is then needed to be recovered with hacks like FiniteElement.facet_support_dofs() and dolfin::DirichletBC::compute_bc_geometric.

Implement FiniteElement.support_entity_dofs().

[Jan Blechta]

It popped out in email discussion that FiniteElement.facet_support_dofs() is not fully equivalent. But its use cases can possibly be covered by suggested FiniteElement.support_entity_dofs().

[Lawrence Mitchell]

FWIW, FIAT.finite_element contains the free function entity_support_dofs which integrates basis functions on the entity to determine which ones have support.

[Prof Garth Wells]

This would also help with the solver-side construction of dofmaps, see https://github.com/FEniCS/ffcx/issues/56.