fix: disallow QQuantity with derived units

[Antyos]

Fixes #339.

Added:

• Units with exponents (1/s, m^2) are supported
• Raises a NotImplementedError when using non-simple units (m/s, N).

There is probably a world where it wouldn't be too hard to infer the prefixes if it's a single unit like N, but I'm not tackling that right now.

I have also updated the docs and added corresponding tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.85%. Comparing base (3285493) to head (e0bd85b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
+ Coverage   84.81%   84.85%   +0.04%     
==========================================
  Files          49       49              
  Lines        3904     3909       +5     
==========================================
+ Hits         3311     3317       +6     
+ Misses        593      592       -1     

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