Fix #30 by adding a csv exporter.

This PR will also come with few changes:

• Introduce the GenericNode abstract class to define what a "node" in a GenericGraph is. This will replace the previous Item type that was just an alias over Any.
• Some changes in the API of the Mapping class.
• Avoid multiple inheritance with classes with different metaclasses on Program. This is related to abstract class instantiable when subclassing built-in types python/cpython#50246
• Clean code and better typing support

Still missing:

• Add documentation in diffing-portal about the new exporter
• Make the csv exporter as the default one in the CLI
• Change the README.md help description