Create a wrapper class that includes data(?), model, and gridsearch

[JulianBiesheuvel]

With a wrapper class, the user can choose what model they would like to use (for now it only accommodates XGBoost but in the future possibly a neural network), accounts for the grid-search (often the model and the grid-search go together), and the data (t.b.d.). It also provides extra functionalities like saving and loading a model from a file, and more...

• different kinds of plotting methods

[JulianBiesheuvel]

The architecture we discussed—having a parent class Model with child classes XGBoostCustom and MLP—is too simple and not feasible. We should consider a more elaborate approach, as I described earlier. For now, I've implemented the custom XGBoost class with all the necessary features. However, features like saving and loading the model and performing grid search should ideally be part of a wrapper class rather than being included in each individual model. Some features that are currently implemented for the CustomXGBoostRegressor (but should be part of a wrapper), are:

• Fix pickling problem with closure in custom loss by using dill #36
• Make an option for the user to choose if the code will be run on either the CPU or GPU #9
• Provide the option for the users to choose between gridsearch or randomsearch #41

For now, this, and the issues listed above, are on hold. In future releases, a wrapper/parent class will be provided that accounts for different kinds of models. For now, the focus is on the XGBoost model.