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
Fix output of VFS on interval
Fix output of HDiv/HCurl fields

The test for if they were CG or not was wrong.
Fix DG coordinate field for periodic interval

Ugly hack until we get fed it from DMPlex.  Note that this is probably
wrong when symbolic geometry stuff lands.
Add some simple div-free tests for extruded intervals