Abstract interface for surrogate models

Description

Define an abstract interface for surrogate models usable in System.make_surrogate, so that users can define their own models.

Main changes

• Make SurrogateModel a pure interface, with abstract methods train and predict.
• Define existing models as concrete implementations of SurrogateModel.
• Implement a proxy pattern in SystemSurrogate, to encapsulate the trained/untrained logic previously coded in SystemSurrogate.

Side changes

• Move module surrogate_models from cosapp.systems to cosapp.utils.
• Refactoring pass in models and test files.