Allow label transform functions

[jjdelc]

No description provided.

[gergness]

Looks good, consider adding an integration test though?

In particular, it wasn't clear to me whether it will consider "No Data" when looking for the common prefix in trim_common. (I think it does the right thing because we access the labels via Dimension.labels which calls self.all_elements.valid_elements

Also note there are a few failing unit tests because of the internal changes.

[ernestoarbitrio]

LGTM

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.94%. Comparing base (26b841d) to head (3164525).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/cr/cube/util.py	84.61%	2 Missing ⚠️
src/cr/cube/dimension.py	97.56%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #424      +/-   ##
===========================================
- Coverage   100.00%   99.94%   -0.06%     
===========================================
  Files           19       19              
  Lines         5127     5176      +49     
===========================================
+ Hits          5127     5173      +46     
- Misses           0        3       +3     

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