Handle variables that have a branding suffix

[mauzey1]

These changes will allow CMOR to process variables that have a "branding suffix" in their table entry name as defined in #762.

There are also minor fixes to error messages for whengrid_label or nominal_resolution are missing from the CV file.

[durack1]

@mauzey1 great to see progress on this. I wonder how much does this depend on the format of CMIP7_CV.json in #778 - I am guessing it doesn't but just wanted to make sure

@taylor13 ping just FYI

[mauzey1]

@durack1 This change will only get the branded variable from the variable tables. It doesn't check the attributes extracted from it against the CV. I can update this PR this week to include checking the CV.

[taylor13]

regarding #779 (comment), I think there are only a few values in the cmor tables that also appear in the CV lists of the CMIP7_CV.json file and I don't think CMOR needs to check those. We can do those checks before putting a value of "approx_interval" or "realm" into a CMOR table.

We should, on the other hand check user input of such things as "source", "frequency", "region", etc. for consistency with the CVs. I think you're already doing that though. Right?

[mauzey1]

@taylor13 This PR will be merged to add the functionality for detecting the branding suffixes in variable names from the table and adding their values as global attributes. I will work on the CMIP7 checks in the next PR.