fix immersed manifold cell orientation doc
attempt to improve FS/VFS scalar-valued vs vector-valued mess
FIAT -> FFC/FIAT
small mixed rewrite
integral spacing fixes
minor extrusion demo changes
fix splelling
Merge pull request #318 from firedrakeproject/doc-fs-fixes

* doc-fs-fixes:
  fix splelling
  minor extrusion demo changes
  integral spacing fixes
  small mixed rewrite
  FIAT -> FFC/FIAT
  attempt to improve FS/VFS scalar-valued vs vector-valued mess
  fix immersed manifold cell orientation doc
Test applications of BCs to mixed system with zero block
Correctly apply DirichletBCs to zero matrix blocks

Previously if we had a zero matrix block we would not execute a kernel,
and hence would not apply any boundary conditions to that block.
Add test for BrokenElement and TraceSpace functionality
update TraceSpace -> TraceElement
Pin down exterior facet trace dofs by applying zero BC
Merge pull request #322 from firedrakeproject/bc_zero_block

* bc_zero_block:
  Correctly apply DirichletBCs to zero matrix blocks
  Test applications of BCs to mixed system with zero block
Merge pull request #320 from firedrakeproject/hybridisation_test

* hybridisation_test:
  Pin down exterior facet trace dofs by applying zero BC
  update TraceSpace -> TraceElement
  Add test for BrokenElement and TraceSpace functionality
Fix mixed bcs test (VFS case was wrong)
Do not silently override COFFEE compiler/isa config

Use environment variable PYOP2_BACKEND_COMPILER and PYOP2_SIMD_ISA as
default if set.
Add extrusion documentation
Add introduction
Add kernel passing details