fix: rectified proxy location accessor behavior

[Helveg]

Describe the work done

Proxy locations had no location accessors. They now return a location accessor that when used warns users that they're not accessing the location they think they might be accessing when they use the 2 public members .section and .mechanisms. The returned section and mechanisms will belong to a "best guess" from among the branches connected to the proxy (which can come from an arbitrarily complex network of downstream proxies and children, and a parent). This proxied location may have different mechanisms than the proxy, so when a KeyError occurs we throw a specialized KeyError that also explains that the mechanism access was proxied.

List which issues this resolves:

closes #214

Tasks

• Added tests
• Updated documentation

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📚 Documentation preview 📚: https://arborize--219.org.readthedocs.build/en/219/

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📚 Documentation preview 📚: https://bsb-neuron--219.org.readthedocs.build/en/219/

[drodarie]

Good to be merged once the mpi4py issue is solved.

[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.78%. Comparing base (0951e81) to head (713bd63).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
libs/arborize/arborize/builders/_neuron.py	97.05%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   78.73%   75.78%   -2.95%     
==========================================
  Files         196       28     -168     
  Lines       18168     1714   -16454     
  Branches     2141      220    -1921     
==========================================
- Hits        14305     1299   -13006     
+ Misses       3308      363    -2945     
+ Partials      555       52     -503     

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