Geocode addresses when processing a source

[ezwelty]

Support for address geocoding is almost finished, but still needs to actually be used as a fallback for output feature geometry in Source.process. Sources where only some features need geocoding:

• Australia > Victoria > Wodonga
• United States > California > West Hollywood
• United States > Rhode Island > Providence

An additional field (e.g. geocoded) could be added by Source.process to document whether a feature's geometry was geocoded from address.

A new method to manage address geocoding and caching for a Source would also be helpful.