Add base model and implement model class resolution

[cthoyt]

Summary

This PR implements a base model for all DDI models where we can keep shared code (it can later be subclassed based on the different tasks or to create meaningful abstractions between models that share concepts). It then uses the class-resolver package for lookup of models, which is how we made the pipeline() function in PyKEEN that I think will go well in this package too.

• Code passes all tests
• Unit tests provided for these changes
• Documentation and docstrings added for these changes

Changes

• Create base model class
• Make all models inherit from it
• Add model resolver

[benedekrozemberczki]

Sorry @cthoyt could you pull again and I will merge!