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.