add DNA/protein check on first 100 bp of FASTA files

[ctb]

One of the most dangerous flaws in our protein hash calculations is that sourmash doesn't do any sequence type checks: you need to explicitly specify --input-is-protein for amino acid inputs, and there is no error checking on that, e.g. see #999 (comment).

En route to bigger changes in the way we do things per
#999, this adds checks for compute to verify proper --input-is-protein behavior.

This does NOT deal with API-level issues like add_sequence and add_protein, this is just about command-line signature compute.

Example output

% sourmash compute -k 31 GCA_001593925.1_ASM159392v1_genomic.fna.gz -o xyz.sig --input-is-protein 

...

WARNING: input is protein, turning off nucleotide hashing
...
... reading sequences from GCA_001593925.1_ASM159392v1_genomic.fna.gz
** ERROR: for filename GCA_001593925.1_ASM159392v1_genomic.fna.gz,
** ERROR: this looks like DNA sequence, but you're using --input-is-protein

and

% sourmash compute -k 31 GCA_001593925.1_ASM159392v1_protein.faa.gz -o xyz.sig 
...

computing signatures for files: GCA_001593925.1_ASM159392v1_protein.faa.gz
...
... reading sequences from GCA_001593925.1_ASM159392v1_protein.faa.gz
** ERROR: for filename GCA_001593925.1_ASM159392v1_protein.faa.gz,
** ERROR: this looks like protein sequence; use --input-is-protein

---

• 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 #1037 into master will decrease coverage by 0.09%.
The diff coverage is 80.48%.

@@            Coverage Diff             @@
##           master    #1037      +/-   ##
==========================================
- Coverage   92.37%   92.27%   -0.10%     
==========================================
  Files          72       72              
  Lines        5454     5492      +38     
==========================================
+ Hits         5038     5068      +30     
- Misses        416      424       +8     

Impacted Files	Coverage Δ
sourmash/command_compute.py	95.20% <80.48%> (-2.44%)	⬇️

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