Undocumented interfaces in config_src/external/Icepack_interfaces

[Hallberg-NOAA]

The code that was imported from Icepack to provide interfaces for SIS2 to use when the Icepack code is not available violates the MOM6/SIS2 coding standards with regards to documentation and how modules are accessed. Although the module use issues can not be addressed without also modifying Icepack, there is no excuse for not adequately documenting the interfaces in the code in the SIS2 repository. Specifically,

1. A large number of subroutine arguments are not described in comments, and none of them use Doxygen syntax.
2. The units of a number of real variables are not documented.
   Both of these should be corrected in the code in confic_src/external/Icepack_interfaces.