Redefining default output representations and metrics

The paper originally described the six conventional tasks, and the alternative method based on the 75-most common Roman numerals.

A Roman numeral can also be reconstructed by predicting the notes conforming the chord. This has become the main approach in this version.

Some other additions include:

• An updated README with instructions for inference and training
• A notebook to run an inference demo
• An updated version of the dataset with the new output representations