Surface more customization options for bulk expression heatmaps in pixie

[givison]

Is your feature request related to a problem? Please describe.

Creating the first heatmap in the section titled "Weighted cell SOM cluster average heatmap over channels (z-scored)" (I think the cell is called "cell_som_heatmap"). The legend is covering a good portion of the actual heatmap

Describe the solution you'd like
Maybe an argument to hide the metacluster annotations? Or some way to shrink the legend.

[alex-l-kong]

@givison thanks for bringing this to our attention, looks like this can become an issue if a metacluster has a very long name. We'll look into it.