provide both abundance-weighted coverage & flattened coverage in `sourmash gather` output?

[ctb]

when looking at #1805, I realized that it would be nice to get both flat and abundance-weighted coverage of metagenomes when running gather. Unfortunately I don't think that's currently tracked internally, so it's a bigger change than just printing something additional out, but it would be convenient.

(Specifically, both the CSV output and the text output should contain this information.)