Make gurobi models pickeable

[cdiener]

Fixes #123.

Before:

In [1]: import optlang

In [2]: import pickle

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

In [4]: x = pickle.dumps(mod)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-82cf2f51c584> in <module>()
----> 1 x = pickle.dumps(mod)

AttributeError: Can't pickle local object 'Configuration._tolerance_functions.<locals>.<lambda>'

After:

In [1]: import optlang

In [2]: import pickle

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

In [4]: x = pickle.dumps(mod)

In [5]: len(x)
Out[5]: 679

[codecov-io]

Codecov Report

Merging #124 into master will decrease coverage by 20.89%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
- 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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