* Add map reference point constants `*_MAP_REF_POINTS `that can be used
for map projections
* Add (unimplemented )`*_project` and `*_sample` methods to the `Maps`
implementation
* Add `PollenSample` and `UviSample` structs
* Make `Position::new` const
* 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)
