Remove cgen from assemble_expressions
Remove cgen from orientation code.
No more cgen in core_types.
Remove cgen from docs and travis
clean up ast use
Don't import *, it clobbers the namespace.
Incomplete expression to ast converter.
Merge pull request #208 from firedrakeproject/remove-cgen

Remove cgen

Replace cgen with the PyOP2 IR everywhere. This also means that all of the kernels which Firedrake produces are purely IR rather than c, except to the extent to which they contain user code.
Fix projection of Expressions

We need to interpolate into L2, not H1, before projecting, otherwise
things come out all wrong if the coordinate field is not H1.