The `RetrievedMaps` struct captures the image and its metadata:
the last modification time and the base timestamp for the maps.
* No longer store the last modification time, called "stamp" before,
separately in the `Maps` struct
* Update methods on `Maps` to use the `RetrievedMaps` structs and
the timestamp base in particular for sampling and map marking
* Update the `MapsRefresh` implemention to use the last modification
time
* Rename some variables from `map` to `image` in the helper functions
for consistency
* Update tests and documentation
* 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
This way we can build Rockets from outside the crate and run benchmarks,
for example.
* Add top-level `setup()` function to create a Rocket and set up the
maps refresher task
* Change the type of `maps::run` since `!` is still an unstable type
* Fix HTTP code blocks in `README.md` so they don't appear as doctests
to rustdoc
