Specify required versions of MOM6, FMS & FMScoupler

[ashjbarnes]

The core library in this package will work with the versions of MOM6 & FMS code specified in MOM6_examples as of Jan 2024. However, the code here is quite old and doesn't include the latest improvements to open boundary conditions, or to surface fluxes (required to use ERA5 forcing). This was made apparent by issues Chris found in issue #79

Things fall over when running our examples though, as the MOM_input and input.nml files need minor changes to reflect versions used.

Need to link specific commits to all three of these codebases, and add some rudimentary instructions for how someone modifies the MOM_examples repo to include updated code. Compiling is another story though and probably beyond the scope of our documentation