Pool struct for primal solutions of colgen

[guimarqu]

Close #912, close #808

[codecov]

Codecov Report

Patch coverage: 93.33% and project coverage change: +1.03 🎉

Comparison is base (d4338da) 69.41% compared to head (99e0c77) 70.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #915      +/-   ##
==========================================
+ Coverage   69.41%   70.45%   +1.03%     
==========================================
  Files          82       83       +1     
  Lines        5918     5929      +11     
==========================================
+ Hits         4108     4177      +69     
+ Misses       1810     1752      -58     

Impacted Files	Coverage Δ
src/MOIwrapper.jl	35.16% <0.00%> (ø)
src/MathProg/MathProg.jl	100.00% <ø> (ø)
src/MathProg/duties.jl	62.50% <ø> (ø)
src/MathProg/pool.jl	94.73% <94.73%> (ø)
src/MathProg/formulation.jl	62.29% <100.00%> (+12.68%)	⬆️
src/MathProg/projection.jl	94.23% <100.00%> (ø)

... and 1 file with indirect coverage changes

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