* Define the map key for Buienradar as `MAP_KEY` (colors used on
Buienradar maps)
* Define a `MapkeyHistogram` type and add the `map_key_historgram()`
function to construct one
* Define the sample size to look for pixels around the sampling
coordinate
* Introduce a separata `sample` function that returns the samples
for a map and the provided coordinates and starting timestamp
* Implement `Maps::pollen_sample` and Maps::uvi_sample`