Unify solving methods

[dimitri-justeau]

The current Python API is quite similar to the Java one. It might be nice to think about a unified solving API for Python, associated with only one documentation. Ideally, it should be easy to :

• Find one solution and get the values of any variable in the model.
• Find and store several solutions, and for each solution store the values of any variable in the model.
• Find one or several optimal solutions, retrieve the best solutions' variables' values, and eventually intermediary solutions.
• ... Any other idea ?

It may be worth it to think about a Pythonic way of doing it, maybe something not possible in Java, or not easily, but much more convenient and easy to remember for users.