[MRG] require scaled signatures for containment

[ctb]

I stumbled across some code that shouldn't have worked while writing a new test for #1374, and discovered that we are allowing contained_by to be called on num MinHash objects.

This is unambiguously wrong and I think we should remove it from the code base. Only scaled MinHashes allow accurate estimation of containment. I guess we didn't know that a few years back, though, so it crept into the code base :).

#1345 is relevant - but while there are reasonable places to call count_common on regular MinHashes, there are no reasonable places to call contained_by, AFAICT.

NOTE: this breaks backwards compatibility, but I think it's rectifies a bug so we're ok.

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?

[codecov]

Codecov Report

Merging #1381 (5b3a8b9) into latest (001cd35) will increase coverage by 5.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1381      +/-   ##
==========================================
+ Coverage   88.84%   94.16%   +5.32%     
==========================================
  Files         123       96      -27     
  Lines       18270    14707    -3563     
  Branches     1409     1410       +1     
==========================================
- Hits        16232    13849    -2383     
+ Misses       1800      621    -1179     
+ Partials      238      237       -1     

Flag	Coverage Δ
python	94.16% <100.00%> (+0.02%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/minhash.py	93.35% <100.00%> (+0.04%)	⬆️
src/sourmash/sourmash_args.py	92.24% <100.00%> (+0.32%)	⬆️
tests/test__minhash.py	99.31% <100.00%> (+<0.01%)	⬆️
tests/test_sourmash.py	99.42% <100.00%> (+<0.01%)	⬆️
src/core/src/wasm.rs
src/core/src/index/bigsi.rs
src/core/src/errors.rs
src/core/src/encodings.rs
src/core/src/ffi/signature.rs
src/core/src/ffi/minhash.rs
... and 22 more

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[ctb]

Ready for review @luizirber @bluegenes

[ctb]

yay, tests pass!

[ctb]

🎉