Fixes Issue #335 (fix from Steven Vandekerckhove)
Add function to return UFC element from FiniteElement.

Intended for low-level usage only.
Fix a bunch of missing member const in GenericFunction.
Merge branch 'garth/fix-missing-const-genericfunction'
Merge branch 'garth/make-ufc-element-protected'
Merge branch 'stevenvdk/fix-issue-335'