add 'hemisphere' option to heatmap()

[carlocastoldi]

Description

This small PR adds the hemisphere option in order to plot only the right, left or both hemispheres. This can be useful when comparing two different but symmetrical data.

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

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f9e7f90) 0.00% compared to head (d0ce373) 0.00%.

Files	Patch %	Lines
brainglobe_heatmap/heatmaps.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #27 +/- ## ===================================== Coverage 0.00% 0.00% ===================================== Files 5 5 Lines 285 285 ===================================== Misses 285 285 ```

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[adamltyson]

@carlocastoldi does this work for you? Trying it out locally I get AttributeError: 'Mesh' object has no attribute 'cutWithPlane' with every example, unless hemisphere="false".

[alessandrofelder]

I get the same as Adam 🤔

[carlocastoldi]

yes, this PR depends on https://github.com/brainglobe/brainrender/pull/304