Split `color` option for edges and add `edge_colors`
Created by: femtobit
This PR supersedes #756 based on the discussion there, where we have deemed compatibility with the v3.0 betas to be more important than making edges
a property. Therefore, this PR keeps that interface while splitting the somewhat overloaded color
option into two separate option (with deprecation warning).
Just as #756, this PR also adds an edge_colors
attribute returning just the list of colors.
Closes #750 (closed).