fix: update COG category mapping

[siddC]

Update COG mapping. We originally used X for genes with no COG category but that is unfortunately incorrect (see https://www.ncbi.nlm.nih.gov/research/cog/cogcategory/X/#). X is actually a COG category which deals with the mobilome (prophages, transposons, and the like).

I've updated the cog_dict in gene_util to reflect this, and added - to represent genes with no COG annotation

[codecov]

Codecov Report

Merging #108 (1c57c39) into master (77537f6) will not change coverage.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   54.23%   54.23%           
=======================================
  Files          17       17           
  Lines        3321     3321           
  Branches      642      643    +1     
=======================================
  Hits         1801     1801           
  Misses       1395     1395           
  Partials      125      125           

Impacted Files	Coverage Δ
src/pymodulon/gene_util.py	0.00% <ø> (ø)
src/pymodulon/core.py	74.80% <50.00%> (ø)
src/pymodulon/plotting.py	87.01% <100.00%> (ø)
src/pymodulon/util.py	69.90% <100.00%> (ø)

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