Create a stub of the icebergs module

[theresa-cordero]

This PR creates stub interfaces for the icebergs module within SIS2 so that when SIS2 is merged into MOM6 the icebergs module does not need to be merged in as well.

[Hallberg-NOAA]

Thank you for this useful contribution, which seems to me to be correct.

[marshallward]

As seen in regression testing, these stubs can unexpectedly displace the calls to an external libicebergs.a if ones mkmf/makedep paths uses a generic form such as config_src/external/*, which I expect is currently very common for most of us.

To prevent this, we may want to add MOM_error() to these stub functions.

[Hallberg-NOAA]

I have modified this PR to issue a fatal error (with an explanatory message) if the new stub code is called.

[Hallberg-NOAA]

We appear to have disabled our ability to directly push changes onto the dev/gfdl protected branch, so I have created a parallel pull request (PR #241) that adds the required fatal error messages if this new stub code is inadvertently used. This PR will be closed once PR #241 is approved and merged in.