Add approval+runoff and blanket primary variants (issue #17)

[endolith]

Latest: irv(..., n_winners=...) (renamed from n_survivors); docstrings use Unified Primary, Top Four vs Final Five, and spell out “general election using the contingent vote” where relevant; top_n_runoff cites contingent vote instead of supplementary vote; removed top_four_* / top_five_* one-line wrappers in favor of top_n_* with n=4 or 5.

  

[what-the-diff]

PR Summary

• Enhanced Election Methods
  We've added new capabilities related to election systems and blanket primaries. New functions include candidate elimination, tiebreaking, and others to provide advanced functionalities, which you can access in the newly developed blanket_primary.py file.
• New File Creation
  The blanket_primary.py file is a fresh addition to our suite. It's equipped with implementations for two-round election systems and provides options for tiebreakers and determining the winner.
• Updated Imports
  To make the election methods from blanket_primary.py readily useful in other parts of our application, we updated the __init__.py file to include these methods.
• Testing for New Functionality
  We've ensured the correctness of the new election systems by creating the test_blanket_primary.py file. This file houses several test cases, aiding in validating the functionality of the new methods.
• Test Method Adjustments
  The existing test_methods.py file has been refined. We've incorporated the new blanket primary methods to ensure that it covers them, thus validating correct error handling for tied elections and mismatched dimensions in approval ballots.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (8a7213f) to head (c1023db).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage   96.31%   97.15%   +0.83%     
==========================================
  Files          19       20       +1     
  Lines         489      632     +143     
==========================================
+ Hits          471      614     +143     
  Misses         18       18              

Flag	Coverage Δ
no-numba	96.51% <100.00%> (+1.01%)	⬆️
numba	91.61% <100.00%> (+2.45%)	⬆️

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