upwind advection / extrusion demo
Mathjax fixes
Merge pull request #317 from firedrakeproject/tutorial-extrusion

* tutorial-extrusion:
  Mathjax fixes
  upwind advection / extrusion demo
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