Update {temp,enth,age}SystemCtx and a regression test (test G).

Most test G errors are a tiny bit smaller, one is very slightly larger.

TO DO: re-run vfnow.py and look at the convergence rate; compare to
older versions of PISM.
Remove fine vertical grid code from IceGrid.
Working on IceGrid.

Removing IceGrid::set_* calls, clean up compute_vertical_levels(),
add (x0, y0) to Create(...) and Shallow(...).
Add "make retest" to re-run tests that failed.
Reduce the number of IceGrid::set_...().