"geometric" method for strong BCs

This pull request implements "geometric" strong BCs. This applies strong BCs to those dofs whose basis functions do not vanish on the relevant boundary. This enables Dirichlet conditions to be applied to DG fields. It may also allow no slip conditions to be applied to HDiv fields and free slip conditions to HCurl fields.

This pull request depends on https://bitbucket.org/mapdes/fiat/pull-request/20

Extruded is not yet supported (due to WIP at the FIAT level)