Test assembling more math functions
Merge pull request #328 from firedrakeproject/test_moar_ffc

* test_moar_ffc:
  Test assembling more math functions
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
enriched OPE doc + whitespace
start on extruded RT example
finish extruded function space exposition. comments welcome.