Automate objective deactivation

[npaterakis]

Currently the user has to deactivate the objective functions in obj_list before returning their pyomo model. If it is forgotten, an error will be raised as soon as a model instance has to be solved. The error message is quite intuitive (Solver interface does not support multiple objectives). However, I think this should be easy to automate for a neater user workflow:

• Add a new method in the Model class
• Call it at the beggining of the solve method in the PyAugmecon class

The models in the tests directory would have to be updated, but I don't expect an error if the user deactivates the objective functions anyway.