Add transform widget

[adamltyson]

Closes #26. I decided to "brand" this widget as brainmapper as it essentially adds the missing functionality of brainmapper to napari.

This also effectively reverses #84, as we now need napari.

I'm struggling to get the tests cache to work, so they're taking much longer than they should.

Docs and a tutorial need to be added, but I'm holding off doing this until this PR is merged, as the docs will depend on the final UI. Basic instructions are:

• Open napari
• Register data with brainreg
• Detect (with any method) points in the raw data that was registered
• Open plugin
• Set "points layer" to be the detected points
• Load the raw data originally registered, and set as "Raw data layer"
• Click "transform points" and select the brainreg output directory when prompted
• The summary results will be displayed in a table, and the transformed cells added as layers. More detailed info can be exported using the new buttons that will appear under the table.

Once this is merged we should:

• Update the docs
• Release a new version of brainglobe-utils
• Update the brainglobe-utils version in the metapackage
• Release the new version of the metapackage

[codecov]

Codecov Report

Attention: Patch coverage is 99.18367% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.68%. Comparing base (78533f8) to head (52aab75).

Files	Patch %	Lines
brainglobe_utils/brainmapper/transform_widget.py	99.01%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   91.59%   92.68%   +1.08%     
==========================================
  Files          35       37       +2     
  Lines        1607     1845     +238     
==========================================
+ Hits         1472     1710     +238     
  Misses        135      135              

Flag	Coverage Δ
numba	92.62% <99.18%> (+1.09%)	⬆️

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[alessandrofelder]

Thanks @adamltyson - looking good.

I made a few nitpicky comments that I think would help with legibility.

I opened an issue to address the cache issues separately #92