should sourmash gather insist on uniform scaling?

[ctb]

thinking through some of the gather issues revealed/discussed in https://github.com/sourmash-bio/sourmash/issues/2950, and also the bug in https://github.com/sourmash-bio/sourmash/issues/2825, and also worrying that branchwater fastgather/fastmultigather don't handle adaptive downsampling properly, I'm wondering if we should insist that either all database sketches have a scaled no higher than the query, or there is an explicit --scaled argument provided?

so, if a query had scaled=1000 and a database sequence had scaled=10,000, gather would refuse to run unless --scaled=10000 was specified.

It seems like an obvious UX improvement and deals nicely with confusing issues revealed in #2825.