Remove regridding over ocean in streams handling

Right now when we read in streams datasets the regridding is done over ocean points as well -- but we ignore the ocean data. So there's a waste in both memory and CPU for handling ocean points that are unused. It likely doesn't matter much, but it's good to be aware of. It was a feature that I didn't fully appreciate before. And it is a source of potential savings in performance if we did decide to work on it, since 2/3rds of the globe is covered in ocean.

The ESMF folks told me the way to to this is "You can tell the regridding to not regrid to particular locations (e.g. the ocean points) by using dstMaskValues in ESMF_FieldRegridStore(". In this case this just means taking advantage of the streams_mask option to shr_strdata_init_from_inline for streams datasets.