[MRG] fix truncation of tax genome classification

[bluegenes]

fix bugs in tax genome:

• continue instead of break (caused genome to truncate classification early)
• when classifying by threshold, use the match rank rather than args.rank

[codecov]

Codecov Report

Merging #1648 (6d8eb5f) into latest (4fce5ab) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           latest    #1648   +/-   ##
=======================================
  Coverage   81.91%   81.91%           
=======================================
  Files         112      112           
  Lines       11447    11447           
  Branches     1432     1432           
=======================================
  Hits         9377     9377           
  Misses       1813     1813           
  Partials      257      257           

Flag	Coverage Δ
python	89.32% <100.00%> (ø)
rust	66.43% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/tax/__main__.py	89.01% <100.00%> (ø)

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