[FEATURE REQUEST] Color scale(s) in the SVG output

[FlorianTrigodet]

The need

When saving an SVG from the interactive interface, we're missing the color scale for any data displayed as "intensity". A typical example is when we have an ANI heatmap on top of a pangenome, it would be great to have a color scale in the SVG output.

The solution

Add a color scale for any data displayed as intensity. The scale would be a gradient from the min to max color and also include the min/max value.

Beneficiaries

There are currently 22 persons attending an anvi'o workshop who find this feature very useful.