fix: Caching morphology loaders during placement

[drodarie]

Description

When running placement strategy jobs, the user can use a distributor to generate morphologies on the fly, this will make the loading of the morphologies performed by each job longer the more morphologies are generated and stored.
The pool_cache decorator used to cache node properties across jobs of the same strategy does not work on the nodes stored as sub-class attributes.

Work achieved

• Cache the morphology loaders across similar placement strategy jobs.
• Fix pool_cache decorator to work with strategy node attributes (by visiting sub classes)

List which issues this resolves:

Close #201 #202

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📚 Documentation preview 📚: https://bsb-core--208.org.readthedocs.build/en/208/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.11%. Comparing base (a8bfccf) to head (5cbc43e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
+ Coverage   78.23%   85.11%   +6.87%     
==========================================
  Files         196       86     -110     
  Lines       17936    10875    -7061     
  Branches     2096     1280     -816     
==========================================
- Hits        14033     9256    -4777     
+ Misses       3350     1331    -2019     
+ Partials      553      288     -265     

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