add multipeek functionality to report equal gather matches

[ctb]

The big CounterGather refactor in #1489 left us with a fairly simple API for reporting gather matches - peek tells you what the next match could be, while consume pulls it off the counter.

However, there's some interest in reporting equivalent matches - see #1366 #278. Unfortunately peek can't do this, as it returns only a single match at a time. I think we could add a function multipeek that returns all of the equivalently scoring matches.

This does leave us the ...interesting question of how to report them...

Other things to consider -

• there are equivalent matches (which is just about reporting) and equivalently sized matches (which represent bifurcating paths in the gather algorithm's choices, that would lead to different lower rank matches). The former is easy to report with a '*' or something, like we did in sourmash gather. What do we do about the latter?? Right now we are just reporting them at random. I don't know how often this happens, though. Maybe we need to study it.

[ctb]

(a suggestion that comes from #1366 is to break ties, maybe by choosing the one with the best max_containment?)