Fix variable of formulation

[guimarqu]

Now, you can fix an active and explicit variable in a formulation.
Fixing a variable means it has the same lower and upper bound and it is then deactivated until one of its bounds changes.

[codecov]

Codecov Report

Base: 86.26% // Head: 86.33% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (e7b7309) compared to base (0a1b549).
Patch coverage: 97.14% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #718      +/-   ##
==========================================
+ Coverage   86.26%   86.33%   +0.06%     
==========================================
  Files          58       58              
  Lines        5724     5751      +27     
==========================================
+ Hits         4938     4965      +27     
  Misses        786      786              

Impacted Files	Coverage Δ
src/MathProg/variable.jl	83.33% <0.00%> (ø)
src/MathProg/formulation.jl	81.98% <100.00%> (ø)
src/MathProg/manager.jl	72.34% <100.00%> (+4.04%)	⬆️
src/MathProg/varconstr.jl	95.36% <100.00%> (+0.40%)	⬆️

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