[MRG] Trial improvements to sourmash argparse output for compute.

[ctb]

This makes sourmash compute return:

new output:

sourmash compute
usage: 

   sourmash compute -k 21,31,51 *.fa *.fq

This will create MinHash sketches at k-mer sizes of 21, 31 and 51, for
all FASTA and FASTQ files in the current directory, and save them in
signature files ending in '.sig'. You can rapidly compare these files
with `compare` and query them with `search`, among other operations;
see the full documentation http://sourmash.rtfd.io/.

The key options for compute are:

 * `-k/--ksize <int>[, <int>]: k-mer size(s) to use, e.g. -k 21,31,51
 * `-n/--num <int>` or `--scaled <int>`: set size or resolution of sketches
 * `--track-abundance`: track abundances of hashes (default False)
 * `--dna or --protein`: nucleotide and/or protein signatures (default `--dna`)
 * `--merge <name>`: compute a merged signature across all inputs.
 * `--singleton`: compute individual signatures for each sequence.
 * `--name-from-first`: set name of signature from first sequence in file.
 * `-o/--output`: save all computed signatures to this file.

Please see -h for more options and more detailed help.

---
 compute: error: the following arguments are required: filenames

rather than

old output:

                [--scaled SCALED] [--protein] [--no-protein] [--dayhoff]
                [--no-dayhoff] [--hp] [--no-hp] [--dna] [--no-dna]
                [--input-is-protein] [--seed SEED] [--input-is-10x]
                [--count-valid-reads COUNT_VALID_READS]
                [--write-barcode-meta-csv WRITE_BARCODE_META_CSV]
                [-p PROCESSES] [--save-fastas SAVE_FASTAS]
                [--line-count LINE_COUNT] [--rename-10x-barcodes FILE]
                [--barcodes-file FILE] [-f] [-o OUTPUT] [--singleton]
                [--merge FILE] [--name-from-first] [--randomize] [-q]
                [--check-sequence] [--license LICENSE]
                filenames [filenames ...]
 compute: error: the following arguments are required: filenames

which I think is an improvement? Open to your thoughts on formatting, content, etc.

Addresses #899

• 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?

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #931 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #931   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files          70       70           
  Lines        4947     4948    +1     
=======================================
+ Hits         4540     4541    +1     
  Misses        407      407           

Impacted Files	Coverage Δ
sourmash/cli/compare.py	100.00% <ø> (ø)
sourmash/cli/gather.py	100.00% <ø> (ø)
sourmash/cli/search.py	100.00% <ø> (ø)
sourmash/cli/compute.py	100.00% <100.00%> (ø)

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