Refactor and clean up code to follow Sklearn convention

[kmzzhang]

• save all parameters and hyperparameters (x, y scales, and network hyperparams) to state dict for simplicity
• Use state_dict keyword in place of featurizer and flow keywords for initializing model
  • this makes it easier to distribute model now that network hyperparams are saved to the state_dict
• use set_params, get_params, save_params method names
• other refactoring

[kmzzhang]

@stefanv I expanded integration tests to include the new features. Request merge