ensure un-sorted hist de-duplication

[mathause]

• See using np.unique to de-duplicate historical ensembles shuffles them #338
• Test files updated
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

This should make the hist ensemble members unique without shuffling them. The test files are not yet updated. I am also not sure if I will merge this into v0.9, or wait for afterwards.

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edit: I decided to create a new version v0.9.1 for this. I don't close the related issue because I want to at estimate the implication of this first.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (616c8f2) 87.88% compared to head (14c0619) 87.95%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #339      +/-   ##
==========================================
+ Coverage   87.88%   87.95%   +0.06%     
==========================================
  Files          40       40              
  Lines        1742     1752      +10     
==========================================
+ Hits         1531     1541      +10     
  Misses        211      211              

Flag	Coverage Δ
unittests	87.95% <100.00%> (+0.06%)	⬆️

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