Add Balas extended formulation for convex hull of H-rep

[blegat]

This PR adds

• Balas extended H-rep of convex hull of union of H-reps
• extended H-rep of intersection of extended H-reps
• JuMP constraints with extended H-reps

[coveralls]

Coverage decreased (-0.1%) to 91.497% when pulling 3c5331f on bl/extended into b61db89 on master.

[codecov]

Codecov Report

Merging #221 into master will decrease coverage by 0.15%.
The diff coverage is 82.55%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
- Coverage   90.88%   90.72%   -0.16%     
==========================================
  Files          34       36       +2     
  Lines        2073     2157      +84     
==========================================
+ Hits         1884     1957      +73     
- Misses        189      200      +11     

Impacted Files	Coverage Δ
src/Polyhedra.jl	84.00% <ø> (+4.00%)	⬆️
src/dimension.jl	86.36% <50.00%> (-3.64%)	⬇️
src/projection_opt.jl	70.37% <70.37%> (ø)
src/elements.jl	83.87% <72.22%> (-1.34%)	⬇️
src/extended.jl	97.43% <97.43%> (ø)
src/opt.jl	83.60% <0.00%> (+4.57%)	⬆️

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[joehuchette]

Good to have this!