Refactoring, reformatting, and testing

[elichad]

• Splits converter.py into three files according to purpose:
  • converter.py for overarching conversion functions
  • crate_utils.py for retrieving values and entities from crates
  • mapping_utils.py for loading mappings and parsing keys
• adds tests for crate_utils.py and mapping_utils.py (i.e. progress on #6)
• fixes #18, and fixes the various flake8 failures that were hidden by that
• adds docstrings, type hints, and renames a few functions, to improve understandability

Does not add any new functionality or change any existing functionality.