cast units to str before applying any other preprocessor

[keewis]

The idea is to accept integer units of 1 for dimensionless, which is explicitly allowed by the CF conventions (we could, however, pass through objects that are not integers unmodified).

Note that pint itself doesn't really like the idea of integer units, so this will only work with ureg.parse_units(1): ureg.Quantity(1, 1) will raise before any preprocessor is called.

• closes an aspect of correct_units fails on CMIP6 historical tos data jbusecke/xMIP#322

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a9cebee) 85.78% compared to head (3ee6b89) 85.79%.

❗ Current head 3ee6b89 differs from pull request most recent head 997ea4b. Consider uploading reports for the commit 997ea4b to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #498   +/-   ##
=======================================
  Coverage   85.78%   85.79%           
=======================================
  Files          13       13           
  Lines        2364     2365    +1     
  Branches      183      183           
=======================================
+ Hits         2028     2029    +1     
  Misses        303      303           
  Partials       33       33           

Flag	Coverage Δ
mypy	38.55% <100.00%> (+0.02%)	⬆️
unittests	93.99% <100.00%> (+<0.01%)	⬆️

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