Fix constraint primal calculation in gurobi interface

[cdiener]

Now gives the correct primals for the constraint expression:

In [1]: import optlang

In [2]: gurobi = optlang.gurobi_interface

In [3]: mod = gurobi.Model()
Academic license - for non-commercial use only

In [4]: x = gurobi.Variable(lb=3, ub=3, name="x")

In [5]: const = gurobi.Constraint(x, lb=3)

In [6]: mod.add([x, const])

In [7]: mod.optimize()
Out[7]: 'optimal'

In [8]: x.primal
Out[8]: 3.0

In [9]: const.primal
Out[9]: 3.0

Fixes #115.

[codecov-io]

Codecov Report

Merging #120 into master will decrease coverage by 20.89%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
- Coverage   89.43%   68.53%   -20.9%     
==========================================
  Files           8        8              
  Lines        2460     2460              
  Branches      542      542              
==========================================
- Hits         2200     1686     -514     
- Misses        171      696     +525     
+ Partials       89       78      -11

Impacted Files	Coverage Δ
optlang/cplex_interface.py	2.13% <0%> (-89.51%)	⬇️
optlang/exceptions.py	88.88% <0%> (-11.12%)	⬇️
optlang/expression_parsing.py	76.82% <0%> (-7.32%)	⬇️
optlang/interface.py	90.11% <0%> (-0.58%)	⬇️

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[KristianJensen]

This works great for <, > and = constraints, but not for range constraints (which gurobi does not support natively). I'll adapt your solution a bit to also cover that.

[cdiener]

Hi is that planned to move to master? I was just hit by the same bug again.

[KristianJensen]

Strange. It was merged in #129

[cdiener]

Okay just saw that I only have that problem if when using the barrier methods in gurobi. Maybe some different interpretation of slack variables or something?