Fix advection term in residual

Since the advection term is (\vec{u}\cdot\nabla) \vec{u}, grad should be replaced by nabla_grad in the weak formulation of the residual.
Added more mathematical details

Added text explaining why nabla_grad is used in this context. Also clarified the contraction of indices in the rest of the text by adding brackets where appropriate.
Add Eike and University of Bath to AUTHORS
Merge pull request #300 from firedrakeproject/fix_burgers_demo

* fix_burgers_demo:
  Add Eike and University of Bath to AUTHORS
  Added more mathematical details
  Fix advection term in residual
Set name for Mesh created from plex
remove use of as_form (removed from UFL)
Merge pull request #304 from firedrakeproject/upstream_compat

remove use of as_form (removed from UFL)
Plausible non-extruded implementation

Include the boundary node discovery method in the BC creation methods.
Currently untested.
Fix error norm for constant solutions

Fix error norm so that Constants and Expressions can be used as the
exact solution.
Geometric boundary conditions test.
Disable geometric boundary conditions for extruded