Refactor scripts for generating TREC-COVID baselines (rounds 3, 4, 5)

[lintool]

Runs remain exactly the same - just refactoring to reduce code duplication.

[codecov]

Codecov Report

Merging #1348 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1348      +/-   ##
============================================
+ Coverage     51.79%   51.84%   +0.05%     
- Complexity      805      809       +4     
============================================
  Files           154      154              
  Lines          8625     8625              
  Branches       1224     1224              
============================================
+ Hits           4467     4472       +5     
+ Misses         3786     3781       -5     
  Partials        372      372              

Impacted Files	Coverage Δ	Complexity Δ
...java/io/anserini/ltr/feature/CountBigramPairs.java	89.61% <0.00%> (+6.49%)	33.00% <0.00%> (+4.00%)

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