Tutorial on how to implement a new architecture

Added

• Tutorial on how to implement a new architecture covering basic and advanced use cases (#188)
• Additional and improved documentation - particularly of implementation details (#188)

Changed (breaking for new model implementations)

• Moved multiple private methods to public methods (particularly logging) (#188)
• Moved get_mask method from BaseModel into utils module (#188)
• Instead of using label to communicate if model is training or validating, using self.training attribute (#188)
• Using sample((n,)) of pytorch distributions instead of deprecated sample_n(n) method (#188)