Refactor to make it an installable package with a CLI

[Crivella]

• Change code structure to a python package
• Add pyproject.toml with required dependencies
• Add CLI to run the code using a JSON as an input file

TODO:

• Modify README to reflect changes
  • Split dependencies in one automatically installed, by required external ones
  • Describe how to run the code through the CLI
• Test compatibility of Solver/precond with different combination of dependencies