TST: include test for multiprocess

[mj-will]

This PR adds a test for the Pool object from multiprocess and updates the docs to mention that nesssi supports it.

Closes #193

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.69%. Comparing base (585fa14) to head (44bd07d).
Report is 74 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #427   +/-   ##
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  Coverage   93.69%   93.69%           
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  Files          71       71           
  Lines        6806     6806           
=======================================
  Hits         6377     6377           
  Misses        429      429           

Flag	Coverage Δ
unittests	93.69% <ø> (ø)

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