Use Abstract models and interfaces instead of optional configurable models

[gerardojbaez]

Will be a better approach to use abstract model classes and model interfaces instead of using real models in the package. This will force the developer to implement their own models that extends and implements the base abstract class.

This is helpful in cases where the developer wants to apply different logics to each model. A good example is the Plan model, for multilingual applications, this model needs to be translated.

The current approach works, but IMHO this new alternative is a better fit.

Any suggestions are welcome!