Add farkas certificate for variable bounds

[rafabench]

Closes #105

[codecov]

Codecov Report

Merging #116 (5ad939e) into master (f864f9a) will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   49.02%   49.41%   +0.39%     
==========================================
  Files           9        9              
  Lines        3572     3592      +20     
==========================================
+ Hits         1751     1775      +24     
+ Misses       1821     1817       -4     

Impacted Files	Coverage Δ
src/MOI/MOI_wrapper.jl	86.78% <100.00%> (+0.19%)	⬆️
src/api.jl	16.45% <100.00%> (+0.13%)	⬆️
src/lib.jl	6.71% <0.00%> (+0.13%)	⬆️

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