add load_cmip_data_all_esms helper function

[mathause]

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

Added load_cmip_data_all_esms helper function.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (74b19c9) 87.42% compared to head (b42f335) 87.47%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
+ Coverage   87.42%   87.47%   +0.04%     
==========================================
  Files          38       38              
  Lines        1678     1684       +6     
==========================================
+ Hits         1467     1473       +6     
  Misses        211      211              

Flag	Coverage Δ
unittests	87.47% <91.66%> (+0.04%)	⬆️

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Files	Coverage Δ
mesmer/calibrate_mesmer/calibrate_mesmer.py	92.92% <100.00%> (+0.42%)	⬆️
mesmer/io/_load_cmipng.py	82.78% <90.90%> (+2.13%)	⬆️

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