Fix infinite recursion in gurobi interface

[cdiener]

Basically the following call stack was possible before:

model.update -> model._remove_variables ->  model.objective._get_expression -> model.variables -> model.update

which is a loop.

Now fixed so Objective._get_expression uses model._variables instead.

Should fix #119.

[codecov-io]

Codecov Report

Merging #121 into master will decrease coverage by 20.89%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- 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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