`GeoJsonDataSource` should have a `process` method similar to CzmlDataSource

[thw0rted]

The current implementation of GeoJsonDataSource does not give any way to dynamically load features in multiple batches, because it always clears the entity collection when load is called. CzmlDataSource offers a process method which simply calls the underlying (private) load method without clearing existing entities. This method should be added to GeoJsonDataSource for consistency.

(I previously had a comment here about doing the same thing for KmlDataSource, but on reflection that doesn't make sense. CZML and GeoJson are based on packets/features and it makes sense to support stream-wise loading; KML is a document format that usually includes e.g. style references and it makes sense to only support loading when the whole thing is available.)

[thw0rted]

I put together a simple PR adding this to GeoJsonDataSource, because it was trivial. I'm reasonably confident that the current implementation will handle processing various kinds of input without inducing any weird behavior. I added one test, to call load with some features, and then process with additional features, and that works. More tests could be added to poke at edge cases but I suspect it's fine. (The team could of course have insight to the contrary!)