Fix logic to determine non-desired variables in split-vars

[ceblanton]

The existing logic used both known patterns and a list of variables whose number of dimensions look like metadata. The problem was that the matching logic was applied to the patterns, which was fine, but also the "short var" list, e.g. "a" and "b". This caused any variable with "b" such as "drybc" to be excluded from the output.

Describe your changes

Issue ticket number and link (if applicable)

Fixes #862 (replace XXX with the issue number and GitHub will autolink the PR to the issue)

Checklist before requesting a review

• I ran my code
• I tried to make my code readable
• I tried to comment my code
• I wrote a new test, if applicable
• I wrote new instructions/documentation, if applicable
• I ran pytest and inspected it's output
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.15%. Comparing base (dea2038) to head (49ba283).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #864      +/-   ##
==========================================
+ Coverage   84.14%   84.15%   +0.01%     
==========================================
  Files          71       71              
  Lines        4975     4981       +6     
==========================================
+ Hits         4186     4192       +6     
  Misses        789      789              

Flag	Coverage Δ
unittests	84.15% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
fre/pp/split_netcdf_script.py	74.30% <100.00%> (+1.11%)	⬆️

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[cwhitlock-NOAA]

So to make sure I'm understanding the problem...

Formula terms like "a" and "b" that have a single dimension were previously getting classified as shortvars_to_exclude and then added to the VAR_EXCLUDE_PATTERNS, which lead to all variables with an a or a b in the string to be classed as metadata-like variables. The solution was to say that we wanted different levels of pattern-matching for the VAR_EXCLUDE_PATTERNS and the shortvars_to_exclude - VAR_EXCLUDE_PATTERNS should be a pattern-matching and shortvars_to_exclude is an exact match.

This looks perfectly reasonable given my understanding of what's going on - I might want to add a bit more documentation on the var classification, but that's a separate PR.