Fix sample data loading to be independent of current working directory

[IgorTatarnikov]

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Description

What is this PR

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

Why is this PR needed? Currently, the sample data could only be loaded if napari was started in the package directory. Otherwise, you'd get a file not found error.

What does this PR do? Allows the sample data to be opened regardless of the working directory.

References

Please reference any existing issues/PRs that relate to this PR.

How has this PR been tested?

Tested locally by visual inspection.

Is this a breaking change?

No.

Does this PR require an update to the documentation?

No.

Checklist:

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.69%. Comparing base (9531886) to head (5b1eb88). Report is 1 commits behind head on main.

Files	Patch %	Lines
brainglobe_registration/sample_data.py	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #47 +/- ## ========================================== - Coverage 81.15% 79.69% -1.47% ========================================== Files 8 9 +1 Lines 382 389 +7 ========================================== Hits 310 310 - Misses 72 79 +7 ```

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