SourceDocument refactor for separate raw field

[edwinzhng]

Addresses #1048

Made the SourceDocument an abstract class so that we can have a separate raw() function that returns content() by default. I also added the respective tests and updated relevant collections.

[codecov]

Codecov Report

Merging #1054 into master will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master    #1054      +/-   ##
============================================
+ Coverage     43.03%   43.06%   +0.03%     
- Complexity      634      638       +4     
============================================
  Files           133      135       +2     
  Lines          8077     8080       +3     
  Branches       1168     1168              
============================================
+ Hits           3476     3480       +4     
+ Misses         4271     4270       -1     
  Partials        330      330              

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/anserini/collection/AclAnthology.java	80.00% <ø> (ø)	3.00 <0.00> (ø)
.../java/io/anserini/collection/BibtexCollection.java	85.71% <ø> (ø)	3.00 <0.00> (ø)
...ain/java/io/anserini/collection/CarCollection.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...va/io/anserini/collection/ClueWeb09Collection.java	44.78% <ø> (ø)	2.00 <0.00> (ø)
...in/java/io/anserini/collection/CoreCollection.java	64.58% <ø> (ø)	3.00 <0.00> (ø)
...o/anserini/collection/CovidCollectionDocument.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...in/java/io/anserini/collection/HtmlCollection.java	62.22% <ø> (ø)	3.00 <0.00> (ø)
...in/java/io/anserini/collection/JsonCollection.java	72.54% <ø> (ø)	3.00 <0.00> (ø)
...io/anserini/collection/NewYorkTimesCollection.java	35.32% <ø> (ø)	3.00 <0.00> (ø)
...n/java/io/anserini/collection/TweetCollection.java	65.69% <ø> (ø)	2.00 <0.00> (ø)
... and 9 more

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[nikhilro]

LGTM 🚢

[lintool]

Ref: #1048