Force ddG color transform color maps to be divergent by default

Maybe we should have a 'divergent' checkbox for color transforms

This will reduce the number of threshold values from 2 to 1 ensuring symmetric normalize around zero
Also perhaps it should filter the available colormaps in matplotlib/colorcet to only supply divergent ones?

Locate the ddG color-transform implementation and its matplotlib/colorcet colormap configuration; the issue names no files or tests. Clarify whether the desired behavior is a divergent default, a checkbox, or filtering available maps, then define completion as symmetric normalization around zero with one threshold and an agreed colormap set.

Vom Indexierungsmodell aus dem Issue-Text verfasst.