added EdgeListVectorizer

[jc-healy]

Added a new class for vectorizing data in the form of row_name, column_name, count triples.
Added some unit tests.
Documentation and an expanded functionality to come at a later date.

[codecov-commenter]

Codecov Report

Merging #88 (629938a) into master (4f0b6ae) will decrease coverage by 0.14%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   91.00%   90.86%   -0.15%     
==========================================
  Files          26       28       +2     
  Lines        4404     4499      +95     
==========================================
+ Hits         4008     4088      +80     
- Misses        396      411      +15     

Impacted Files	Coverage Δ
vectorizers/_vectorizers.py	83.58% <ø> (ø)
vectorizers/edge_list_vectorizer.py	77.61% <77.61%> (ø)
vectorizers/__init__.py	100.00% <100.00%> (ø)
vectorizers/tests/test_edge_list_vectorizer.py	100.00% <100.00%> (ø)

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