Documentation!

[MFJansen]

We need to work on documentation, both in the form of comments in the code, and with a separate document. MOM6 is using Doxygen, which automatically creates documentation from annotated source code. Could that be an option for us? (I have no experience with it.)

[MFJansen]

I Added some comments in the code. These will serve as minimal documentation for the time being but eventually a proper documentation should be included.