Expose Placemark.properties

[frederoni]

Fixes #159

I've split up Placemark and GeocodedPlacemark into separate files to improve readability.

The actual changes are:

- superiorPlacemarks = try container.decodeIfPresent([GeocodedPlacemark].self, forKey: .superiorPlacemarks)
+ superiorPlacemarks = try container.decodeIfPresent([Placemark].self, forKey: .superiorPlacemarks)

along with exposing Placemark.properties, which I don't think should be exposed after a second thought because we have aliases to most of these properties. 🤔

cc @1ec5 @riastrad