Rework on packing factor in Particle placement.

[drodarie]

Rework packing factor computation to take into account

• cases where counts and arrays of counts are provided
• voxels ids provided in particle_types

Added tests and documentation.

[codecov[bot]]

Codecov Report

Merging #762 (61ccab5) into main (7bdf19b) will decrease coverage by 0.29%. The diff coverage is 96.96%.

:exclamation: Current head 61ccab5 differs from pull request most recent head 6f8e6c1. Consider uploading reports for the commit 6f8e6c1 to get more accurate results

@@            Coverage Diff             @@
##             main     #762      +/-   ##
==========================================
- Coverage   73.76%   73.48%   -0.29%     
==========================================
  Files         134      134              
  Lines       15852    15875      +23     
==========================================
- Hits        11694    11666      -28     
- Misses       4158     4209      +51     

Files	Coverage Δ
tests/test_placement.py	88.10% <100.00%> (-11.90%)	:arrow_down:
bsb/placement/particle.py	73.09% <87.50%> (+3.09%)	:arrow_up:

... and 6 files with indirect coverage changes