Reduce DOLFIN dependencies in generated code

[garth-wells]

It would simplify a number of matters if generated (JIT) code did not depend on DOLFIN. See #102.

[TormodLandet]

If this is dolfin JIT and not ffc JIT then please remember that a major use for dolfin JIT is to extend dolfin. Is not a major point of dolfinx is to slim down dolfin and make it easier to create add-ons instead of putting all the functionality in the dolfin repo?

A common use of dolfin JIT for me is to write C++ functions that accept dolfin::Mesh or dolfin::Function etc, but potentially also sub-classing dolfin:: classes and adding functionality that way. All of these will require linking with libdolfin to compile (I think ...). One of the strength of FEniCS is to be able to write 95% of the code in Python and then easily JIT the rest as C++ extensions. No complicated building or linking steps and very fast turn around time when iterating on some user code

[garth-wells]

This is the FFC, and hopefully DOLFIN managed, JIT (e.g., simple Expressions in the latter case where only a simple function call is needed).

For user compiled code we can pass responsibility for manage dependency versions to the user - DOLFIN could never handle this comprehensively so any attempt would be half-baked. Custom user JIT code is a top priority in DOLFINX.

[garth-wells]

This is in pretty good shape now. FFC generated code doesn't depend on DOLFIN.