suzerain: #2998 leave zero-zero modes

Leave the mean fields in place so that the ensemble mean
can be removed during postprocessing.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44738 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #2998 remove TODO

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44739 0bd85f0b-c151-43b9-a1e9-27231104551f