Python vault refactoring and code organization

[Esgrove]

Making a separate PR for my Python changes made when implementing the init and update for Rust.

• In Vault.py, put private functions and methods after public. Change helper functions to static methods inside Vault class.
• Encapsulate STATIC_IV as a static class member since it is not used outside the Vault class.
• Make to_str public since it is used in the cli
• Streamline vault initialization for init case and others