revisit LinearIndex.select behavior for scaled (and num?)

[ctb]

in this review, @bluegenes pointed out that LinearIndex.select(...) was not necessarily correct -

I like the approach of selecting sigs that could be converted! ...but not all scaled sigs can be downsampled (desired scaled < sig scaled), right?

and I said I'd put together a test. But my current hot take is that the way we do downsampling in the code base, two scaled signatures will always be downsampled to the higher scaled value, so there is never a situation where select(...) will fail on two scaled sequences. This is particular to LinearIndex and other signature collections, though; SBT and LCA indices cannot always be downsampled.

anyway, this is something I want to revisit! maybe as part of more/better tests, #1427.

[ctb]

from #1524,

A few design decisions were made as part of this - the most consequential one is that select just selects compatible signatures, and doesn't actually do any downsampling or anything. See #1072 (comment) for links.

This has proven to be a good design decision, I think, because we have situations where we do or may want access to the original sketch. Closing this issue.