HOMOGENIZE_FORCINGS does not rescale properly for large factors

[Hallberg-NOAA]

HOMOGENIZE_FORCINGS does not rescale properly for sufficiently large rescaling factors. In particular, for sufficiently large or small rescaling factors, these can return updated values of 0 or something that is too large to be represented, but for modest rescaling factors it gives the right answers.

The issue is that the homogenize_field() routines (approriately) use the reproducing sums via the global_area_mean() routines to give domain-independence and rotational invariance, and the reproducing_sums() routines in turn use the extended fixed point arithmetic.

The solution would seem to be to temporarily rescale the variables inside global_area_mean() back to mks units while taking the spatial means before restoring the scaling for the result. To accomplish this, I am proposing a new optional argument, tmp_scale, to global_area_mean(), global_area_mean_u() and global_area_mean_v() in MOM_spatial_means.F90, and a similar argument to the various homogenize_field rouitines in MOM_forcing_type. The appropriate rescaling factor would also have to be added to all of the calls to these routines in homogenize_forciing() and homogenize_mech_forcing().