Give a summary of the key changes

• Closes #1
• Addresses #2

Either:

• This PR has no merge dependencies, or
• Depends on: ..., or
• Should be merged before: ...

• OpEn version ...
• opengen version ...

• API documentation updated
• Appropriate testing
• Update CHANGELOG(s)
• Update webpage documentation, if necessary
• Bump versions (in CHANGELOG, Cargo.toml and VERSION), if necessary