Completions: Allow string input for date format

The `init` method for the `Completion` class should be converted to allow string input for the `date_format` parameter.

- The code should automatically convert this string to the correct Enum entry when setting the attribute value.
- If no date format has been provided, the code should default to the Nexus default.
- The docstring should also be updated to add this parameter in, and specify that either string or enums are allowed.
- New tests / test cases should be added to test this new functionality.