Add support for direct loops to par_loop.
Merge pull request #324 from firedrakeproject/direct-parloops

* direct-parloops:
  Add support for direct loops to par_loop.
Check mapping before interpolation
Merge pull request #323 from firedrakeproject/interpolate-check

* interpolate-check:
  Check mapping before interpolation
No longer defeat cache in compile_form

* For split forms, the kernels were stashed on the last split form
  instead of the original form
* The check for returning stashed kernels compared the kernel prefix
  with the full kernel name including integral type etc.
Add FFC interface test for mixed form
Test that op2 Solvers are properly collected
Add extrusion documentation
Add introduction
Add kernel passing details
Add link to variational problems documentation
Add details on extruded mesh construction
Add Function Space documentation
move file
add preamble
fix off-by-one references
initial extruded function space rewrite
more extruded fspace doc
rearrangement
OPE doc introduction