Refactor Python tolerance handling

- Set tolerances for different units separately/independently
- e.g. setting an angular tolerance shouldn't clear the current length tolerance
- Convenience functions like `Tolerance.meters(1e-9)`, `Tolerance.degrees(1e-3)`, `Tolerance.unitless(1e-12)` etc.?
- also generic ones like `Tolerance.length(Length.micrometers(10))` etc.
- Have default tolerances pre-configured? 1e-9 meters, radians, and unitless...
- middle ground: `Tolerance.defaults()` to explicitly set up default tolerances for everything

No files, tests, or entry points are named. Start by locating the Tolerance type and its unit-specific handling, then clarify which of the independent setters, convenience functions, and defaults are required; the issue is done only once that scope and its expected behavior are agreed.

Written by the indexing model from the issue text.