Brine plume changes, passing info from SIS2 to MOM6

[breichl]

• Adds necessary code in the FMS coupler to allocate the ice_ocean_boundary field "excess_salt". This is needed to use the brine plume parameterization that relies on an additional information exchange between SIS2 and MOM6.
• Adds necessary code in the FMS coupler to pass the salt_left_behind from SIS2 to the ice_ocean_boundary excess_salt field, making it available for use in MOM6.

[breichl]

I assume the test is probably failing because it uses a version of MOM6 prior to the addition of excess_salt to the ice_ocean_boundary type. I have tested this with the dev/gfdl versions of MOM6 and SIS2 and confirmed it works in ice_ocean mode.