Issue with So_t field under ATM+DOCN configuration

@jedwards4b @mvertens I think there is something wrong in the following piece of code in CMEPS,

https://github.com/ESCOMP/CMEPS/blob/04e2f6bc98959ec9b062de8fde06197c8a64c0e1/mediator/esmFldsExchange_cesm_mod.F90#L1364C11-L1364C93

In my case, I am tying to run WRF+DOCN configuration (CMCC regional coupling work) and it seems that So_t is not passed to WRF correctly. The mediator history file shows that field read in from DOCN correctly (ocnImp_So_t looks fine) but atmExp_So_t has all zeros. Initially, I think that maybe something wrong with the mask values but I could not find any issue over there. So, I changed it like following,

if (fldchk(is_local%wrap%FBexp(compatm), 'So_t', rc=rc)) then
call addmap_from(compocn, 'So_t', compatm, mapnstod_consf, 'ofrac', ocn2atm_map)
call addmrg_to(compatm, 'So_t', mrg_from=compocn, mrg_fld='So_t', mrg_type='copy')
end if

and now atmExp_So_t looks fine. BTW, i am using mapnstod_consf but maybe this could be mapbilnr_nstod. mapconsf does not fill the coastline since we have land-sea mask difference between atmosphere and ocean.

Anyway, I am not sure why this is not catched before. Maybe the configurations were always using land and ice too (even with data ones) and atm+docn is not tested under CEMS framework. Let me know what you think. This is just one line fix and I could create a PR about it. I am not sure this requires testing or not.

BTW, it seems this is correct in nems side,

https://github.com/ESCOMP/CMEPS/blob/04e2f6bc98959ec9b062de8fde06197c8a64c0e1/mediator/esmFldsExchange_nems_mod.F90#L240

Also, we might need to correct masking for the interpolation. At this point following masking is used from ocean to atmosphere,

```
(module_med_map: med_map_routehandles_initfrom_field) creating RH consd for ocn to atm srcMask = 0 dstMask = -987987
```

but maybe we need to mask out land in the atmosphere side side this is SST. So maybe we could set dstMask in ocean->atmosphere configuration to 0 (land) to prevent extrapolation through land like following (atmExp_So_t) field after interpolation. It is fine over ocean but it looks weird over land.


To fix it, I could add following

```
if (n1 == compocn .and. n2 == compatm) then
srcMaskValue = 0
dstMaskValue = 0
endif
```

to `med_map_routehandles_initfrom_field()` call under cesm section. This will fix the field like following,


in this case, the extrapolation also affects lake in WRF side but this is masking issue in WRF cap and I'll fix it.

I am not sure this changes will lead baseline change or not. I don't think since this configuration seems not tested under CESM. Maybe there is no need to play with mask and we could go with the original field (extrapolated also over land). Anyway, let me know what you think.