Allow not doing the pullback within uflacs

[Dominic Kempf]

As briefly touched during the discussions in Oslo, it would be nice to not have uflacs always do the pullback. Other backends (say: dune-pdelab) can do the pullback through high level abstractions easily, while the "manual" pullback performed by uflacs produces cluttered code.

[Martin Sandve Alnæs]

This shouldn't be a problem. I'm working on merging work on piola mappings and pullbacks in ufl, updating uflacs to match. Modified terminals will be allowed to be either in reference or global frame, and not doing symbolic pullbacks just a matter of dropping that step.