Improve NGram training

[TsumiNa]

Closes #75 and Closes #86

[codecov]

Codecov Report

Merging #87 into develop will increase coverage by 0.08%.
The diff coverage is 80%.

@@             Coverage Diff             @@
##           develop      #87      +/-   ##
===========================================
+ Coverage    83.04%   83.13%   +0.08%     
===========================================
  Files           56       56              
  Lines         3940     3943       +3     
===========================================
+ Hits          3272     3278       +6     
+ Misses         668      665       -3

Impacted Files	Coverage Δ
xenonpy/inverse/iqspr/modifier.py	85.4% <80%> (+1.49%)	⬆️

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