Purge uses of _ufl_cell
Re-implement ufl_cell

Also ensure that _ufl_cell is never read from except in ufl_cell().
Fix UFL intersphinx
Merge pull request #392 from firedrakeproject/ufl_cell_fix

* ufl_cell_fix:
  Fix UFL intersphinx
  Re-implement ufl_cell
  Purge uses of _ufl_cell
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.