Add a new example visualising brainmapper cells in specific brain regions.

[adamltyson]

I put this together for #381, so I figured it may as well be an example.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (2723b6d) to head (2ca5a9c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
- Coverage   87.77%   86.71%   -1.07%     
==========================================
  Files          27       27              
  Lines        1219     1219              
==========================================
- Hits         1070     1057      -13     
- Misses        149      162      +13     

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[IgorTatarnikov]

Looks good, just one nitpicky question