Remove obsolete example (Green's function kludge).

Remove examples/greensfns/hex8. This example was a kludge that Charles
put together before we implemented the user friendly
FaultCohesiveImpulses.
Add fault edges to examples/2d/greensfns.
Update comments in pylithapp.cfg.
Fixes for clamped edges.

Still have a bug related to buried edges and custom preconditioner.