Make all ffc regression tests pass with uflacs

[Martin Sandve Alnæs]

Issues have been created for separate pieces of missing functionality to fix. Add new specific issues if found, and close this when all tests pass.

[Martin Sandve Alnæs]

Getting all ffc regression tests passing is a good milestone to work towards for feature completeness. The ffc regression tests are not covering all features and corner case, but they do cover most main functionality.

[Martin Sandve Alnæs]

The goal is to make this test pass:

python test.py --print-timing --permissive --ext-uflacs --ignore-code-diff

Most of the ffc regression tests now pass.

Currently failing:

• CustomIntegral -- missing custom integrals implementation
• CustomMixedIntegral -- missing custom integrals implementation
• CustomVectorIntegral -- missing custom integrals implementation
• MetaData -- preprocessing now differs so uflacs cannot be mixed with quadrature in one form

New failures that used to work...:

• PoissonDG.ufl
• Biharmonic.ufl

[Martin Sandve Alnæs]

Moved issue to ffc