[Feature]: Support arbitrary regions

[forsyth2]

Is your feature request related to a problem?

While we primarily want to support standardized regions for analysis, it may be useful to researchers to be able to define their own regions for ad-hoc analyses (e.g., diagnosing issues in a certain region in a high-resolution model).

Describe the solution you'd like

Add the ability for users to specify user-defined regions in the cfg rather than needing to actually change the e3sm_diags source code in https://github.com/E3SM-Project/e3sm_diags/blob/main/e3sm_diags/derivations/default_regions.py.

The primary complication I foresee with this is that regions is specified as a list of strings that default_regions.py then maps to actual regions, so the question is how to specify a region in a cfg. Perhaps it could be parsed from a string (e.g., latitude=(0.0, 45.0, "ccb"), longitude=(210.0, 310.0, "ccb") => "0.0,45.0,ccb,210.0,310.0,ccb")? At that point, it seems like another parameter would have to be introduced, something like user_defined_regions=.

Describe alternatives you've considered

• Keeping standards defined in https://github.com/E3SM-Project/e3sm_diags/blob/main/e3sm_diags/derivations/default_regions.py and not directly supporting arbitrary regions.
• Keep https://github.com/E3SM-Project/e3sm_diags/blob/main/e3sm_diags/derivations/default_regions.py and ask users to update the source code there if they want e3sm_diags to support a new region. This risks bloating the set of standard regions.

Additional context

No response