CRM-19784: Encapsulated interactions with geocoding providers. Adopts an

CRM-19784: Encapsulated interactions with geocoding providers. Adopts an
approach like the factory pattern PHP developers will find familiar, with
plenty of deprecation notices re the changing approach. A specific goal
was to avoid fatal errors in which a geocoder that no longer exists is called.
This applies specifically to the Yahoo class but also generally, e.g., if an
extension that provides a geocoder goes missing due to deletion, changed paths,
etc.