Fix units in ocean models...

... and elsewhere. Updated boundary model tests pass. Also update units
in the forcing manual and fix variable attributes in surface models.

All atmosphere models still use "m / s ice equivalent" as precipitation
units (for convenience). The goal of #16 is to make it possible to do
coupling without matching ice densities, not get rid of ice density in
all "boundary models".

I need to take one more look at all this and then I can close #16.