Implement FJR

[DominikPeters]

Add checks for Full Justified Representation (brute force and gurobi).

In the process, added some extra property tests (check that logical implications and contrapositives hold on instances).

[codecov-commenter]

Codecov Report

Merging #66 (f20b12d) into master (2da4003) will increase coverage by 0.03%.
The diff coverage is 93.54%.

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@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   91.24%   91.28%   +0.03%     
==========================================
  Files          11       11              
  Lines        3141     3200      +59     
  Branches      989     1006      +17     
==========================================
+ Hits         2866     2921      +55     
- Misses        161      163       +2     
- Partials      114      116       +2     

Impacted Files	Coverage Δ
abcvoting/misc.py	87.50% <50.00%> (-1.87%)	⬇️
abcvoting/properties.py	95.08% <96.55%> (+0.19%)	⬆️

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