fd*.yaml file names should correspond to coupling_mode

[jedwards4b]

If coupling_mode is xyz I would like the yaml file to be named fd_xyz.yaml this would make the logic of determining supported coupling_modes in buildnml more automated and will allow users to add new coupling modes more easily without the need to change buildnml.

[DeniseWorthen]

The current singe fd_nems.yaml can be used for any nems coupling mode (the modes differ only in mapping type or in the use of the mediator fluxes in prep_ocn). Making redundant copies of the yaml file would mean keeping multiple files up-to-date vs one file.