Election functions complete w tests

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Added election functions for Limited, Bloc, SNTV, SNTV-STV Hybrid, and Top Two, along with a testing file

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@jamesturk @jgibson517 PR is now updated with end of week objectives. Here are some assorted comments:

• Added graphs.py file which contains PairwiseComparisonGraph object. This will likely get folded into a file with the ballot graph, or share a subfolder with it. Added tests for all methods in test_graphs.py.
• Added metrics.py file which contains only borda_scores, a function that computes a dictionary of borda scores for an election. This belongs in a class of general utility methods to get data about any preference profile we want (ie first place votes, borda scores, etc). It will probably wind up with a different name under the metrics subfolder. Added tests_metrics.py.
• Added election systems for DominatingSets and CondoBorda (using PWC graph) into election_types.py, along with new tests for those in election_tests_2.py
• Slightly modified election_state.py code so that it doesn't require results be lists of strings, it now allow lists of, well, anything, but relevantly sets which will track ties in the outcome (we should likely fully transition outcomes to be sets to allow ties and mirror Ballot at some point)

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@jamesturk @jgibson517 @realdavemccune Made changes in line with James' comments, the most extensive of which are a merge and rewrite of the remove_cand/remove_cand_sets functions, a rewrite of the PreferenceProfile eq function, along with corresponding tests, and the addition of a Ballot eq function