Small formatting fixes

[adamltyson]

• Capitalises classes (closes [Feature] Refactor to have PEP8 class names. #41)
• Adds an extra ruff rule (seems like a good idea to add them to small codebases before it becomes onerous to add)
• Uses pathlib in the cell count example
• Removes another leftover script in a submodule

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 0.00%. Comparing base (55a698b) to head (18b0f2b).

Files	Patch %	Lines
brainglobe_heatmap/planner.py	0.00%	2 Missing ⚠️
brainglobe_heatmap/__init__.py	0.00%	1 Missing ⚠️
brainglobe_heatmap/heatmaps.py	0.00%	1 Missing ⚠️
brainglobe_heatmap/slicer.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #42   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          5       5           
  Lines        282     279    -3     
=====================================
+ Misses       282     279    -3     

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