Aggregate some statistics in node / edge tables

[fedarko]

It'd be really nice to add an extra row (presumably at the bottom?) of the selected node / edge tables that aggregates any numerical metadata that the selected nodes / edges have.

Most of the time, I imagine we could show averages -- e.g. for selected nodes this'd be the average length, GC content, coverage, etc. Probably folks will also want us to show medians (in the case where one or two nodes have outliers or something), sums (e.g. if we select all nodes in a component and want to see the total length of these nodes), etc. However, just starting off with averages should be good enough for a first pass.