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
Add short section in docs on geometric BC method
Merge pull request #299 from firedrakeproject/enhanced_bcs

Geometric determination of boundary condition nodes (useful for applying
strong bcs in discontinuous spaces).

* enhanced_bcs:
  Add short section in docs on geometric BC method
  Disable geometric boundary conditions for extruded
  Geometric boundary conditions test.
  Fix error norm for constant solutions
  Plausible non-extruded implementation