Small formatting fixes

[adamltyson]

• Capitalises classes (closes #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[bot]]

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 :warning:
brainglobe_heatmap/__init__.py	0.00%	1 Missing :warning:
brainglobe_heatmap/heatmaps.py	0.00%	1 Missing :warning:
brainglobe_heatmap/slicer.py	0.00%	1 Missing :warning:

Additional details and impacted files

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

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