Add ax arg to plot method

[RobertoDF]

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

I would like to be able to do something like this:

fig, axs = plt.subplots(1,3, figsize=(15,4))
for ax, d in zip(axs,[8800, 9300, 9700]):
    scene = bgh.Heatmap(
        regions,
        position=(d),
        orientation="frontal",  # or 'sagittal', or 'horizontal' or a tuple (x,y,z)
        thickness=10,
        title="frontal",
        format="2D",
    )
    scene.plot(ax=ax)

Is there a better way to achieve this?

Thanks!

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
• [ ] The code has been formatted with pre-commit

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (8d45490) to head (e8610a2). Report is 16 commits behind head on main.

Files	Patch %	Lines
brainglobe_heatmap/heatmaps.py	0.00%	13 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #44 +/- ## ===================================== Coverage 0.00% 0.00% ===================================== Files 5 5 Lines 279 284 +5 ===================================== - Misses 279 284 +5 ```

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[IgorTatarnikov]

Done and done! I think this is good to merge.