Major revision.  Implemented a more robust control of the final timestep size, spread over the last two timesteps as suggested by others.  This naturally breaks most of the regression tests (i.e. those tests that rely on historically computed solutions rather than necessarily correct ones).  Since they were to be phased out anyway, ALL REGRESSION TESTS HAVE NOW BEEN DELETED.  The tests mainly exercised combinations of two different relperm relations (quadratic vs. Corey--Brooks) and relperm modification options (unmodified vs. modified with Sweby limiter).  These can be conveniently paired up in the MMS folders, since a problem with any option should destroy convergence of the whole folder.  The unmodified and modified relperm options have been assigned to darcy_impes_mms_p1_2phase_quadraticrelperm and darcy_impes_mms_p1_2phase_coreyrelperm, respectively.  Note that the _coreyrelperm folder still needs to have residual saturation and capillary pressure implemented.  There will need to be another MMS folder to replace the deleted three-phase tests.  The pseudo-one-phase folders are less important as they include analytic tests.
It seems the modification to the darcy schema a few revisions back caused the diamond validation test to fail.  Revoking the modification until a more robust equation of state can be implemented (c.f. trunk).