WIP: Fix `sig kmers`

[ctb]

Fixes sig kmers to properly ignore bad DNA unless --check-sequence is provided; adds tests. Fixes https://github.com/sourmash-bio/sourmash/issues/2842.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d6994a1) 86.23% compared to head (6dcc515) 29.74%.

Files	Patch %	Lines
src/sourmash/sig/__main__.py	0.00%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           latest    #2856       +/-   ##
===========================================
- Coverage   86.23%   29.74%   -56.50%     
===========================================
  Files         130      130               
  Lines       14756    14756               
  Branches     2622     2404      -218     
===========================================
- Hits        12725     4389     -8336     
- Misses       1731    10167     +8436     
+ Partials      300      200      -100     

Flag	Coverage Δ
hypothesis-py	25.80% <50.00%> (ø)
python	?
rust	50.88% <ø> (ø)

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[ctb]

ref https://github.com/sourmash-bio/sourmash/issues/3339