Don't throw away supporting entities of dofs

[chrisrichardson]

Original report by Jan Blechta (Bitbucket: blechta, GitHub: blechta).

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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().

[chrisrichardson]

Original comment by Lawrence Mitchell (Bitbucket: wence, GitHub: wence).

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I note that the Strang-Fix scheme (the canonical one) is a symmetric quadrature rule, whereas the four-point collapsed quadrature is non-symmetric. This may or may not be an issue.