Ensure that (un)translated IDs are never `None`

[rsundqvist]

• Update README.md and CONTRIBUTING.md
• Multiple signature updates to ensure that there's always a valid Format to fall back on.
• Moved default str constants to the Format class + convenience methods showcased in docstring
• Bonus content: maximal_untranslated_fraction performance uplift and

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.88235% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 96.71%. Comparing base (c25dd2c) to head (ea3ff91).

Files	Patch %	Lines
src/id_translation/offline/_magic_dict.py	67.85%	7 Missing and 2 partials :warning:
src/id_translation/offline/_format_applier.py	71.42%	2 Missing :warning:
src/id_translation/_uuid_utils.py	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #228 +/- ## ========================================== - Coverage 96.94% 96.71% -0.24% ========================================== Files 60 60 Lines 3506 3530 +24 Branches 600 604 +4 ========================================== + Hits 3399 3414 +15 - Misses 70 78 +8 - Partials 37 38 +1 ```

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