Model specific mask values

I would like to move away from hard coding mask values based on the coupling_mode (see code in med_map_mod.F90). Several applications use multiple compset configurations, each would require a new 'coupling_mode' setting. I think esmFld.F90 variables such as compocn_omask, compocn_lmask, compatm_lmask, etc make the most sense and these variables can be configured by the app_mode files (e.g. esmFldsExchange_cesm_mod.F90). The defaults should be the 'ispval_mask' value.

**Current Code Example**
```
if (trim(coupling_mode) == 'cesm') then
dstMaskValue = ispval_mask
srcMaskValue = ispval_mask
if (n1 == compocn .or. n1 == compice) srcMaskValue = 0
if (n2 == compocn .or. n2 == compice) dstMaskValue = 0
else if (coupling_mode(1:4) == 'nems') then
if (n1 == compatm .and. (n2 == compocn .or. n2 == compice)) then
srcMaskValue = 1
dstMaskValue = 0
```