Object-oriented strategies interface

[bstellato]

Implement strategies with an abstract class. Each strategy will define useful information to help solving the problem and hopefully making it convex. Examples are (combinations of):

• Tight constraints (or subset of)
• Integer variables (or subset of)
• Continuous variables (or subset of)

The object-oriented interface will deal with how to:

1. Retrieve the strategy from the original problem solution
2. Solve the problem with the strategic information

In order to do so, we need to inject the Strategy in the reduction chain (possible after the ParamProg).