Clarify how priorities affect subsampling

[huddlej]

Description of proposed changes

Moves the --priority argument into the appropriate argument group (it only works as a subsampling argument), correct the existing help text to show that the input should be tab-delimited, and expand the help text to clarify how subsampling uses the given priority scores.

The output from augur filter -h for priorities now reads:

  --priority PRIORITY   tab-delimited file with list of priority scores for
                        strains (e.g., "<strain>\t<priority>") and no header.
                        When scores are provided, Augur converts scores to
                        floating point values, sorts strains within each
                        subsampling group from highest to lowest priority, and
                        selects the top N strains per group where N is the
                        calculated or requested number of strains per group.
                        Higher numbers indicate higher priority. Since
                        priorities represent relative values between strains,
                        these values can be arbitrary. (default: None)

Related issue(s)

Fixes #622

Testing

Tested by CI.

[codecov]

Codecov Report

Merging #695 (4f30017) into master (8df4b4d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #695   +/-   ##
=======================================
  Coverage   30.52%   30.52%           
=======================================
  Files          40       40           
  Lines        5615     5615           
  Branches     1363     1363           
=======================================
  Hits         1714     1714           
  Misses       3830     3830           
  Partials       71       71           

Impacted Files	Coverage Δ
augur/filter.py	45.84% <100.00%> (ø)

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