ENH: add core_nt Kraken 2 DB to the list of allowed databases

[misialq]

Closes #223.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.64%. Comparing base (b6d068a) to head (687b46c).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
+ Coverage   95.60%   95.64%   +0.04%     
==========================================
  Files          34       34              
  Lines        1956     1975      +19     
  Branches      226      229       +3     
==========================================
+ Hits         1870     1889      +19     
  Misses         48       48              
  Partials       38       38              

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