Changing a variable or constraint name is now immediately reflected in any containing Container.

Solvers are not update yet though. Need to intercept name changing at the object level.
Removed weird printing of variable IDs if Container would not contain variable
fixed retrieval of integer variables
fixed bug on retrieving objective
Changing variable names is now reflected in the glpk solver instance
Fixed an issue with printed constraints not showing bounds == 0
Fixed reference result for test_optimize_milp
Merge branch 'devel' into fix_mixed_integer_opt
Fixed a test issue due to the updated constrain printing
Merge branch 'devel' into fix_mixed_integer_opt
Merge pull request #5 from biosustain/fix_mixed_integer_opt

fixed retrieval of integer variables
Merge branch 'devel'
Updated setup.py to v0.0.4