Correctly render the default mesh colour in napari

Description

What is this PR

[x] Bug fix
[ ] Addition of a new feature
[ ] Other

Why is this PR needed? Most meshes are coloured white when you add them to napari

What does this PR do? Performs the right colourspace conversion from the structures's RGB to napari rgb, so the mesh default colours are displayed correctly.

References

Closes #50

How has this PR been tested?

Manually:

Also, added a simple unit test.

Is this a breaking change?

Nope

Does this PR require an update to the documentation?

Docstring of _add_mesh has been updated.

Checklist:

[x] The code has been tested locally
[x] Tests have been added to cover all new functionality (unit & integration)
[x] The documentation has been updated to reflect any changes
[x] The code has been formatted with pre-commit

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.41% :tada:

Comparison is base (78bf071) 93.10% compared to head (669d9c6) 93.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #51 +/- ## ========================================== + Coverage 93.10% 93.52% +0.41% ========================================== Files 9 9 Lines 377 386 +9 ========================================== + Hits 351 361 +10 + Misses 26 25 -1 ``` | [Files Changed](https://app.codecov.io/gh/brainglobe/brainglobe-napari/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe) | Coverage Δ | | |---|---|---| | [...c/brainglobe\_napari/napari\_atlas\_representation.py](https://app.codecov.io/gh/brainglobe/brainglobe-napari/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe#diff-c3JjL2JyYWluZ2xvYmVfbmFwYXJpL25hcGFyaV9hdGxhc19yZXByZXNlbnRhdGlvbi5weQ==) | `100.00% <ø> (ø)` | | | [...e\_napari/tests/test\_napari\_atlas\_representation.py](https://app.codecov.io/gh/brainglobe/brainglobe-napari/pull/51?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe#diff-c3JjL2JyYWluZ2xvYmVfbmFwYXJpL3Rlc3RzL3Rlc3RfbmFwYXJpX2F0bGFzX3JlcHJlc2VudGF0aW9uLnB5) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/brainglobe/brainglobe-napari/pull/51/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe)

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brainglobe / brainrender-napari