Add transform widget

[adamltyson]

Closes https://github.com/brainglobe/brainglobe-utils/issues/26. I decided to "brand" this widget as brainmapper as it essentially adds the missing functionality of brainmapper to napari.

This also effectively reverses https://github.com/brainglobe/brainglobe-utils/pull/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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/brainglobe/brainglobe-utils/pull/88/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe) | Coverage Δ | | |---|---|---| | [numba](https://app.codecov.io/gh/brainglobe/brainglobe-utils/pull/88/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe) | `92.62% <99.18%> (+1.09%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe#carryforward-flags-in-the-pull-request-comment) to find out more.

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