This starts to address #14 but didn't turn into a full MR yet.
* Use crates `assert_float_eq` and `assert_matches` for extra assertions
* Split off a function to build a Rocket `rocket()` that can be used
in the tests
* Also cache address geocoding requests to OSM Nomatim!
* Use the `cached` crate for an easy implementation
* Add the `cache_key` helper function to deal with floats being annoying
* Cache Buienradar get request for 5 minutes (per position/metric)
* Cache Luchtmeetnet get request for 5 minutes (per position/metric)
* Note the `Item` structs need to implement `Clone` now because
the cache will own them and Rocket will want a copy too
* Add the `csv` crate as a dependency
* Use the `Row` struct as intermediate object
* Turn the `parse_value` function into the `convert_value` function that
cannot fail
* Replace the lazy `once_cell` by a maps handle type
* Use Rocket's managed state to manage a handle
* Ensure that the handlers have access to it
* Pass another handle to the maps updater loop
* Try to keep the lock as short as possible
Still, long downloads block the lock. Add a FIXME to refactor this
so the lock is only taken when updating the maps fields.
Paul van Tilburg