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.
Added missing PYLITH_CHECK_ERROR(err).
Fix error trapping for PETSc functions.

Since PETSc is now usually built with C compiler, we can't trap errors
using PetscError(). Throw C++ exception from PYLITH_CHECK_ERROR instead.
Fault: Fix definition of fault Section
- Must constrain clamped vertices
- We get the vertices from the label, translate to fault numbering, and constrain all dofs
Fix some small bugs in settings constraints in fault section.
Merge remote-tracking branch 'origin/master' into knepley/fix-faults-parallel

* origin/master:
  Small fixes for clamped edges in cohesive cells for FaultCohesiveDyn. Added full-scale test.
  Turn off journal output.
  Temporary fix for change in PETSc interface.
  Trivial change (formatting).
  Add notes on how to delete tags.
Fault: We need a consistent closure for clamped cohesive cells
- We get the two end faces of the prism and concatenate their closure
Fix for settings constraints for fault fields in parallel.

Need to account for processes without vertices when setting constraints
for fault fields.
Finish full-scale test for spontaneous rupture with embedded fault.
Add TestShearDispFriction to test suite.
More fixes to fault field constraints.

Need to segregate vertices from edges in applying constraints for field
over vertices.
Update full-scale test to use split fields and custom fault preconditioner.
Fixed filtering of vertices/edges in setting constraints for fault fields.
Added use of split fields for 2-D embedded fault test case.
Create nodeset for fault edge in 2-D subduction example.
Fix typo in parameter file.
Fix typo in README.