KmerMinHash::merge in src/core/src/sketch/minhash.rs is leaving the abundance vectors in an inconsistent state when merging two MinHash objects, one with track_abundance=True and one without. This PR adds tests for several but not all of the conditions that aren't being handled, and also proposes some (probably incorrect) Rust-level code fixes.

I think a better solution might be to simply declare this kind of merge to be not allowed, and to force people to flatten the abund MinHash if they want to merge with a no-abund MinHash.

@luizirber your input solicited!

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?