This reverts commit 4f25d438.

We previously used bool() on the constraints expression to find
out whether we need to use the DIrichletConstraintsAssembler.
However, UFL does the stupid thing of evaulating

bool(eq(x[0], 0))

to False, because x[0] and 0 are obvioulsy not the same. Veeeery
bad behaviour for a symbolic language.

We do this from multiple subdirs

Calculate the evaluation and jacobians of geometry mappings of cells
using sum factorization. By avoiding all geometry mapping of intersections
we can avoid the face-twist problem and just make the grid consistens (not
yet done).

TODO:
- Make grid consistent
- Vectorization
- 3D