Packaging, entrypoints and licence

• Add standard LHCb GPLv3 licence + pre-commit check
• Add pyproject.toml and move the source to src/HepDataExtractor
• Turn scripts into modules and move argparse functionality to a single function in __main__.py
  • Add an entrypoint gethepdata that points to this function
  • Script names become commands, but with shorter names: extract_tables, tex_to_yaml (unchanged), prepare_submission
  • Update README and GitLab CI config to reflect this change

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Related to lhcb-simulation/lhcb-simulation-project-website#5

Closes #1 (closed)