Implement contig depth colorization

[fedarko]

From @fedarko on June 10, 2017 10:36

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).

Copied from original issue: fedarko/MetagenomeScope#217

[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]

Closing in favor of #243. (Support for coloring nodes / edges by arbitrary metadata could just be extended to support coverages.)