If the sample/item series are empty, the function already returns
`None`, so the tuple values are always `Some(_)` which makes the
`Option` type redundant.
* Make the combined provider keep track of the AQI and pollen maximum
value
* Extend the `Forecast` struct with the `aqi_max` and `pollen_max`
fields
* Fill the `aqi_max` and `pollen_max` fields when the PAQI metric is
selected
* Update the documentation
* Extend the tests
* Adapt `retrieve_image` to also return a timestamp based on the
CDN's last modified time; adapt other methods accordingly
* For the maps module, use `chrono::Utc` instead of
`tokio::time::Instant` and use `chrono::Duration` instead of
`tokio::time::Duration`
* Pass the maps timestamp to the `sample` function so it can use
that timestamp as base
* Use `Position` everywhere instead of latitude/longitude float values
* Implement `Partial`, `Eq` and `Hash` for `Position` so it can
part of a cache key
* Drop the `cache_key` helper function
* Rename the `address_position` function to `resolve_address`
* Add methods on `Position` for formatting latitude/longitude with
a given precision (used for URL parameters in providers)