Clearer distinction in utils between what is specific to hires diags vs what is more general

[mnlevy1981]

We've talked about putting CaseClass in its own repository, and I think a good step to take before going that far would be to split the python modules in utils/ into general CESM utilities (like CaseClass) and tools that are specific to POP / the hires package. I don't know exactly what to include in the latter... maybe the split would involve writing a generic interface for things like global mean / sum in the general utilities but then there's a pop-specific call that knows the weights are TAREA over ocean and 0 over land? I also need to take a closer look at the plotting functions, off the top of my head I don't think we've written them in a POP-specific manner but I could be mistaken.

[matt-long]

Some POP-specific things may belong in pop-tools.