command-line plugin idea: contig gather

[bluegenes]

We have a working contig-level gather implementation in charcoal, but there are applications for this besides contamination detection and removal. It would be neat to write a sourmash plugin that would run a contig-level gather workflow on any query + database.

Briefly, the charcoal approach is:

1. sketch the query as a whole
2. prefetch against the full database
3. using prefetch matches as a new, reduced database, go through each contig and run gather to find the best match per contig
4. summarize/ report

[ctb]

ref #2592

[ctb]

note that multigather is effectively contig gather ref #2328 (comment)