Code cleanup (removes remains of the code supporting PETSc 3.2). See #156.
Regression tests using "extra execs" are disabled by default. See #159.

The tests are still there and *are* used if Pism_BUILD_EXTRA_EXECS is
set to "ON" or if Pism_BUILD_TYPE is set to "Debug" (which implies
Pism_BUILD_EXTRA_EXECS == "ON").