* 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
* Don't set the `homepage` field in `Cargo.toml` if it is the same as
the repository (C-METADATA)
* Don't use `expect` but `?` in examples (C-QUESTION-MARK)
(This cannot be fixed for the README for now, unfortunately)
* Document the errors for all methods which return a `Result`
(C-FAILURE)
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
Also make all of its fields private.
This prevents parsing/building a valid geo URI and then making it
invalid by setting the latitude/longitude to something incorrect.
Paul van Tilburg