Distinguish between abstract Expression and a FE function

[garth-wells]

DOLFIN doesn't clearly distinguish between:

• An Expression that can be evaluated for an x, and
• a FE/quadrature function that can be used in a Form. (See https://bitbucket.org/fenics-project/dolfin/issues/355/expression-s-interpolate-to-linear).

We should have:

• An abstract Expression class (given x, returns a value(s)), and
• A way to put the Expression into a FE/quadrature space.

The problem leads to differences in how a coefficient is evaluated inside a form integral and how is it is evaluated for some x. It is particularly confusing on the Python side.

[garth-wells]

Fixed by https://github.com/FEniCS/dolfinx/pull/229.