Force ddG color transform color maps to be divergent by default

[Jhsmit]

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?

[Jhsmit]

Also, color bars are inverted for some of the quantities (dG, ddG)