Fix handling of ambiguous nucleotides

[ctb]

This is a direct merge combining swamidass#2 + #138.

Addresses #137.

This PR updates behavior for non-ACGT nucleotides to the following:

• k-mers containing non-ACGT are skipped when sourmash compute is run from the command line.
• add_sequence raises an exception on k-mers containing non-ACGT, unless force is True.

• 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-io]

Codecov Report

Merging #148 into master will decrease coverage by 0.07%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
- Coverage   88.28%   88.21%   -0.08%     
==========================================
  Files          18       18              
  Lines        2348     2342       -6     
  Branches       51       52       +1     
==========================================
- Hits         2073     2066       -7     
  Misses        267      267              
- Partials        8        9       +1

Impacted Files	Coverage Δ
sourmash_lib/kmer_min_hash.hh	90.28% <87.5%> (-0.88%)	⬇️

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[betatim]

Thanks for this!