Add filefisher dependency

[veni-vidi-vici-dormivi]

Add filefishes dependency to facilitate loading several scenarios and ensemble members for #537

• Fully documented, including CHANGELOG.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.43%. Comparing base (72d83a5) to head (a32d77d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #586   +/-   ##
=======================================
  Coverage   78.43%   78.43%           
=======================================
  Files          49       49           
  Lines        3056     3056           
=======================================
  Hits         2397     2397           
  Misses        659      659           

Flag	Coverage Δ
unittests	78.43% <ø> (ø)

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