Allow different sketches in the same multisketch to have different kmer sizes

[bovee]

This would require some kind of extension to how we store sketches (neither the Mash binary format nor the JSON format support "Multisketches" with different sketching parameters). This is mostly useful for sketching algorithms that can use sketches of different kmer sizes to impute things about the core vs accessory genomic distance (i.e. see https://github.com/johnlees/PopPUNK), but it's possible there are other uses.