cmap in GuideAssignment.heatmap hardcoded

[JohnGoertz]

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cmap is currently hardcoded to "viridis" in GuideAssignemnt.plot_heatmap: https://github.com/scverse/pertpy/blob/main/pertpy/preprocessing/_guide_rna.py#L377

This should probably be removed, allowing sc.pl.heatmap (and, by extension, plt.imshow) define the default. Alternatively, use kwargs.set_default(cmap, ...). In the latter case, it should probably be a white/light-dark cmap like magma_r, Blues, or YlGnBu, which I would believe to be generally better for visualizing sparse data like this.

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[Zethson]

Makes sense. Would you be willing to make a PR for that, please?