* Introduce and document the `url` feature
* Implement `From<&GeoUri>` and `From<GeoUri>` for `Url`
* Implement `TryFrom<&Url>` and `TryFrom<Url>` for `GeoUri`
* Add and extend tests
The uncertainty distance is a positive floating number.
* Validate this while parsing/building/using the setter
* Add the error variant `Error::OutOfRangeUncertainty` and change
`Error::InvalidDistance` to `Error::InvalidUncertainty`
* Update the tests