Refactor population factory to remove plotting logic and make it a p…

[lfierce2]

This PR refactors the HISCALE population builder to separate population construction from analysis/visualization workflows.

hiscale_observations.py is now a pure builder that ingests observational datasets and constructs a ParticlePopulation. Plotting and validation workflows have been moved to a dedicated example notebook.

[codecov]

Codecov Report

❌ Patch coverage is 96.72131% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.53%. Comparing base (43046ee) to head (8f7d4a4).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/part2pop/analysis/particle/factory/mass_dry.py	82.35%	3 Missing ⚠️
src/part2pop/analysis/particle/factory/mass_tot.py	82.35%	3 Missing ⚠️
...part2pop/analysis/particle/factory/abs_crossect.py	94.11%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #23       +/-   ##
===========================================
+ Coverage   80.64%   92.53%   +11.89%     
===========================================
  Files          65       74        +9     
  Lines        4299     4476      +177     
===========================================
+ Hits         3467     4142      +675     
+ Misses        832      334      -498     

Flag	Coverage Δ
unittests	92.53% <96.72%> (+11.89%)	⬆️

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