[MRG] Fix sourmash prefetch for multiple ksizes in database

[ctb]

When poking around with MAGsearch, I found a problem where sourmash prefetch did not properly apply the ksize learned from an input query to a database via select, which resulted in an incompatible MinHash objects error (see below).

This PR fixes this by selecting the database using the ksize from the query, and adds a test for same.

error message

% sourmash prefetch tests/test-data/GCF_000005845.2_ASM584v2_genomic.fna.gz.sig tests/test-data/GCF_000005845.2_ASM584v2_genomic.fna.gz.sig

== This is sourmash version 4.2.5.dev19+g3a6028fb.d20220217. ==
== Please cite Brown and Irber (2016), doi:10.21105/joss.00027. ==

WARNING: no output(s) specified! Nothing will be saved from this prefetch!
selecting default query k=31.
loaded query: GCF_000005845... (k=31, DNA)
all sketches will be downsampled to scaled=1000
loading signatures from 'tests/test-data/GCF_000005845.2_ASM584v2_genomic.fna.gz.sig'
Traceback (most recent call last):
  File "/Users/t/miniconda3/envs/py37/bin/sourmash", line 33, in <module>
    sys.exit(load_entry_point('sourmash', 'console_scripts', 'sourmash')())
  File "/Users/t/dev/sourmash/src/sourmash/__main__.py", line 13, in main
    return mainmethod(args)
  File "/Users/t/dev/sourmash/src/sourmash/cli/prefetch.py", line 69, in main
    return sourmash.commands.prefetch(args)
  File "/Users/t/dev/sourmash/src/sourmash/commands.py", line 1222, in prefetch
    for result in prefetch_database(query, db, args.threshold_bp):
  File "/Users/t/dev/sourmash/src/sourmash/search.py", line 523, in prefetch_database
    for result in database.prefetch(query, threshold_bp):
  File "/Users/t/dev/sourmash/src/sourmash/index/__init__.py", line 255, in prefetch
    for sr in self.find(search_fn, query, **kwargs):
  File "/Users/t/dev/sourmash/src/sourmash/index/__init__.py", line 172, in find
    shared_size, total_size = query_mh.intersection_and_union_size(subj_mh)
  File "/Users/t/dev/sourmash/src/sourmash/minhash.py", line 571, in intersection_and_union_size
    raise TypeError("incompatible MinHash objects")
TypeError: incompatible MinHash objects

[codecov]

Codecov Report

Merging #1866 (882f080) into latest (186fc1f) will increase coverage by 7.97%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1866      +/-   ##
==========================================
+ Coverage   82.37%   90.34%   +7.97%     
==========================================
  Files         119       89      -30     
  Lines       12934     8733    -4201     
  Branches     1727     1727              
==========================================
- Hits        10654     7890    -2764     
+ Misses       2016      579    -1437     
  Partials      264      264              

Flag	Coverage Δ
python	90.34% <100.00%> (+<0.01%)	⬆️
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/commands.py	88.33% <100.00%> (+0.01%)	⬆️
src/core/src/lib.rs
src/core/src/ffi/minhash.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/ffi/index/revindex.rs
src/core/src/storage.rs
src/core/src/sketch/hyperloglog/mod.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/index/sbt/mod.rs
src/core/src/encodings.rs
... and 21 more

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[ctb]

@sourmash-bio/devs ready for review and merge!

[bluegenes]

lgtm