update 'num' behavior, in terms of downsampling and comparison checking

[ctb]

see #856 (comment) - there,

ctb

in src/core/src/sketch/minhash.rs, the if ignore_abundance code in similarity seems duplicative with the compare function; this should be resolved in some way, or at least commented.
is there a reason not to have SourmashError::MismatchNum be checked in check_compatible (used by lib.kmerminhash_compare)?

luizirber:

These two are related: when I made #808 I had SourmashError::MismatchNum in check_compatible and merged similarity and compare, but that broke the code because check_compatible was not used in any of the scaled/downsampled parts of the code (which obviously don't have the same num.

[ctb]

well, good news is that when you add MismatchNum checking in check_compatible, you only break 6 tests, all in test__minhash.py. Not sure if that's because we're not testing different 'num' minhashes from the command line, or what!

[ctb]

...looks like we're not testing it via the command line 😆

[ctb]

#1955 adds in some code for this downsampling.