consider SourmashSignature downsample and flatten

[ctb]

After #1610, it seems like an obvious next set of simplifications is to remove all of the with x.update(): ... code blocks and replace them with flatten and/or downsample calls.

The only thing that prevents me from doing that now is that I'm not sure the abstraction is at the right level. Do we want to be making detailed calls about what to do with MinHash-specific information like abundances and scaled values, or do we want to be asking for "compatible" sketches on SourmashSignature objects, ref #616?