Display a node length histogram chart/etc. in "assembly information"

[fedarko]

This could be cool.

If this is going to be just one chart for the entire assembly, regardless of whatever the user does in the viewer, then I figure we could just generate the chart in collate.py and store the resulting image in the .db file. However, if we want the user to be able to actually modify the chart dynamically (e.g. to only view information for selected nodes' lengths) or if we want to generate many possible charts (e.g. a histogram of edge multiplicities), then it would make sense to generate the chart in the browser using something like d3.js.

I don't want to use too many dependencies in the project, but d3 shouldn't be too bad to use here.

[fedarko]

This issue was moved to marbl/MetagenomeScope#20