Refactor package to prepare for distribution

[hegdevinayi]

This PR reorganizes the package contents and makes a few minor refactors in the code to ready it for packaging + distribution.

• All files necessary for a local install (e.g., using pip install -e .) have been added
• Absolute import paths are used everywhere to ensure the installed package is used
• Base model is now an abstract class
• A sample test scaffold is added as a placeholder for full-fledged unit tests