Iso19115 indexer, major refactoring and new test cases added

[shaneding]

No description provided.

[codecov]

Codecov Report

Merging #1317 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1317      +/-   ##
============================================
+ Coverage     51.56%   51.60%   +0.04%     
  Complexity      797      797              
============================================
  Files           154      154              
  Lines          8589     8592       +3     
  Branches       1216     1217       +1     
============================================
+ Hits           4429     4434       +5     
+ Misses         3792     3790       -2     
  Partials        368      368              

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/anserini/collection/Iso19115Collection.java	75.00% <100.00%> (+2.27%)	2.00 <0.00> (ø)
...java/io/anserini/ltr/feature/CountBigramPairs.java	64.93% <0.00%> (-1.30%)	23.00% <0.00%> (-1.00%)
...anserini/ltr/feature/base/PMIFeatureExtractor.java	86.53% <0.00%> (+1.92%)	13.00% <0.00%> (+1.00%)

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