remove ensemble anomaly type 'first'

[mathause]

• Closes #xxx
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

This removes ref["type"] == "first" i.e., caculating the anomaly w.r.t. the first ensemble member. Also adds more tests to the data loading and cleans the code/ comments a bit.

[codecov-commenter]

Codecov Report

Base: 86.73% // Head: 86.86% // Increases project coverage by +0.13% 🎉

Coverage data is based on head (4ec66ea) compared to base (f5b36fb).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #247      +/-   ##
==========================================
+ Coverage   86.73%   86.86%   +0.13%     
==========================================
  Files          38       38              
  Lines        1530     1530              
==========================================
+ Hits         1327     1329       +2     
+ Misses        203      201       -2     

Flag	Coverage Δ
unittests	86.86% <100.00%> (+0.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mesmer/io/_load_cmipng.py	80.76% <100.00%> (+1.53%)	⬆️

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