Implement contig depth colorization

[fedarko]

Would be similar to GC colorization, but since depth isn't an absolute measure we'd probably want to implement some form of relative scaling here (e.g. how we handle edge multiplicities).

[fedarko]

I guess we could do scaling on the python side (potentially accounting for outliers using Tukey-esque methods) and assign each node a depth_intensity field or something (between 0 and 1; analogous to the thickness attribute of edges), which could then be easily used as an argument to the getNodeColorization() function, which'd be called when assigning a data(depth_color) attribute for each node in renderNodeObject(). (If that isn't too long a chain of silly stuff.)

That actually should be pretty easy to implement, honestly.

[fedarko]

This issue was moved to marbl/MetagenomeScope#54