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.
Initial par_loop draft.

Initial sketch of Firedrake level par_loop.

Needs documentation, testing and debugging.
And now with passing test.
Add extruded test
Documentation
Response to reviewers

* Remove naughty and evil import *
* Spell RW and C correctly.
Merge pull request #213 from firedrakeproject/parloops

Parloops

This branch creates Firedrake level par_loops. This facilitates things like slope limiters and "Physics". See the extensive documentation below for more details.
Merge pull request #215 from firedrakeproject/fix/projection

Fix projection of Expressions
Add support for extruded facet integrals
Tidy existing tests, and un-xfail
Laplace and Poisson tests for facets, using Neumann BCs
Add interior facet stress tests (P0 and P1DG steady advection)