move fort.gauge on a restart

[rjleveque]

When restarting, fort.gauge files will generally be overwritten. We should first move
to fort.gauge.DATETIME so all such files can be cat'ed together at the end.

This is done in the run_with_restart.py script now found in clawpack/apps/checkpt_restart/run_with_restart, see clawpack/apps#58.

Something similar can probably be incorporated into clawpack/clawutil/src/python/clawutil/runclaw.py

See discussion on clawpack/amrclaw#137.