Remove burgers demo

This demo is broken and not used as a regression test. Prevent
further bitrot by removing it.
Remove stupid MPI demo
Remove adv_diff_mpi demo

This demo is hard to maintain since it relies on pickled objects that
need updating whenever the relevant class changes. Prevent further
bitrot by removing it.
Remove regression tests

No more need for demo and demo/meshes to be packages.
README: remove mentions of regression tests
Remove advection-diffusion demo
Remove non-split advection-diffusion demo
Remove laplace demo
Remove 2D mass demo
Remove 2D mass MPI demo
Remove 2D mass triangle demo
Remove vector mass demo
Remove weak bcs demo
Remove FFC interface
Remove compatible FFC version
Repurpose pyop2-clean to remove cached compiled libraries

Now that the ffc interface is gone, pyop2-clean doesn't need to remove
cached kernels anymore.  However, we do need a way to blow away compiled
libraries which we now cache ourselves.
README: remove FFC, FIAT, UFL dependencies
Remove FFC dependencies from requirements and install.sh
Merge pull request #305 from OP2/remove-ffc-interface

Migrate FFC interface to Firedrake