GeoJson fixes and cleanup

1. Automatically set `perPositionHeight` to true if a polygon contains altitude values.
2. Remove `sourceUri` parameter from the geometry functions since they are no longer used (and haven't been for a while).
3. Move polygon creation code into a helper function to de-duplicate code that was in both `processPolygon` and `processMultiPolygon`
4. Added specs and updated CHANGES.