Implement named dimensions

[blegat]

The LPHRep use a MathOptInterface model to encode the H-representation.
As this model natively supports naming the variables, LPHRep actually supports naming the dimensions.
This PR allows to retrieve these, pass these names along through transformations and sets these names to JuMP variables if the H-representation is used to create a vector of JuMP variables.

[coveralls]

Coverage decreased (-1.0%) to 90.791% when pulling 407def5 on bl/dimension_names into 7144333 on master.

[codecov]

Codecov Report

Merging #225 into master will decrease coverage by 1.30%.
The diff coverage is 68.22%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   90.79%   89.48%   -1.31%     
==========================================
  Files          37       38       +1     
  Lines        2215     2311      +96     
==========================================
+ Hits         2011     2068      +57     
- Misses        204      243      +39     

Impacted Files	Coverage Δ
src/tmp.jl	0.00% <0.00%> (ø)
src/extended.jl	92.85% <83.33%> (-4.58%)	⬇️
src/lphrep.jl	88.09% <91.66%> (+0.25%)	⬆️
src/opt.jl	82.89% <92.85%> (-0.72%)	⬇️
src/default.jl	84.84% <96.00%> (+1.86%)	⬆️
src/center.jl	96.00% <100.00%> (ø)
src/projection_opt.jl	65.51% <100.00%> (-4.86%)	⬇️
src/repop.jl	90.52% <100.00%> (+0.52%)	⬆️

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