* Check that merging fails if the samples/items are too far apart
* Check that nothing is returned if either of the lists is empty
* Check that if either series is shifted, they are merged correctly
* 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