Add dataset resolver around dataset loader class

[cthoyt]

Summary

Similarly to #18, this PR adds base classes for each dataset and a class resolver for looking them up.

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

Changes

• Add derived classes for each dataset
• Add resolver for datasets

[benedekrozemberczki]

Thank you @cthoyt!