Automatic color assignment rule for categorical metadata

[dtriand]

A new rule should be added for varying the color of the points in the RoPE space window based on the value of a categorical variable. Currenly, this is only possible via the change icon rule, but 1. there're only 8 icons to choose from and 2. the user has to manually add rules for every value. The suggested rule should determine the number of unique values and assign each of them a unique color from a colormap. A similar rule can be implemented for an automatic symbol assignment.