FIX: polish the bias analysis

[nocollier]

• Add site extraction for products like Fluxnet2015
• Expand unit conversions to handle units like precip kg m-2 s-1 --> mm d-1
• Settle on a method for calendar trimming
• Create a comprehensive set of tests for CMIP6 models / all dataset types

[codecov]

Codecov Report

Attention: Patch coverage is 81.11888% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 81.33%. Comparing base (56d8121) to head (39cdeb6).

Files	Patch %	Lines
ilamb3/dataset.py	85.93%	9 Missing ⚠️
ilamb3/compare.py	73.33%	8 Missing ⚠️
ilamb3/regions.py	52.94%	8 Missing ⚠️
ilamb3/analysis/bias.py	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   81.24%   81.33%   +0.09%     
==========================================
  Files          11       12       +1     
  Lines         629      777     +148     
==========================================
+ Hits          511      632     +121     
- Misses        118      145      +27     

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