Separate artifacts from wrappers

[Oxid15]

• Adds save_artifact/load_artifact method to each model to save standalone artifacts separately from cascade wrappers
• Model's save method now accepts only folders
• Regular save methods now save only wrappers, the same with load
• Adds ModelLine.load method that allows loading models with and without artifacts
• getitem now loads only wrappers
• Line saves models in the folders and to save artifacts creates artifacts folder inside of them
• Trainer was also updated accordingly

Important part of it is that those changes are breaking previous repo structure and loading models from old repos seems to be impossible since 0.13.0