Combine TreeSurrogate and LocalModel into SurrogateModel

Create a new class `SurrogateModel` which alllows both the estimation of a global surrogate model and a local model.
Maybe TreeSurrogate and LocalModel can also be child classes of SurrogateModel.

If no `x.interest` is provided, automatically estimate a global model. Allow weighting also for the global model.
If `x.interest` is provided, estimate a local model.

Allow both decision tree and linear models for the surrogate. Ideally make it completely flexible and possible for the user to selet a model.

Question: Should it be allowed to provide mutliple `x.interest` and return multiple models?