[MRG] fix some bugs in lca rankinfo.

[ctb]

This fixes two bugs in rankinfo caused by the revised LCA database format in #537, which now permits hashes with no lineage ID.

Bug 1: rankinfo wasn't properly ignoring hashvals with no lineage ID!
Bug 2: if no hashvals were found, rankinfo was failing with a divide-by-zero error.

This also adds an (in-testing) option, --minimum-num, to only count hashvals that occur that many times (or more).

• 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 #797 into master will decrease coverage by 0.14%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
- Coverage   89.38%   89.24%   -0.15%     
==========================================
  Files          29       29              
  Lines        4608     4613       +5     
  Branches       49       49              
==========================================
- Hits         4119     4117       -2     
- Misses        485      492       +7     
  Partials        4        4

Impacted Files	Coverage Δ
sourmash/lca/command_rankinfo.py	90.74% <93.33%> (-0.93%)	⬇️
sourmash/signature_json.py	96.21% <0%> (-1.09%)	⬇️
sourmash/lca/lca_utils.py	95.21% <0%> (-0.8%)	⬇️
sourmash/lca/command_index.py	90.26% <0%> (-0.45%)	⬇️
sourmash/sbt.py	85.21% <0%> (-0.32%)	⬇️
sourmash/sbt_storage.py	86.36% <0%> (+0.97%)	⬆️

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