Force processing sources to implement data source grouping

[mslukebo]

There are two changes required:

1. Force every processing source to implement IDataSourceGrouper. In the spirit of Use base classes instead of interfaces for most types in the SDK namespace #395, this interface should just be removed and the method should just be added directly to the processing source class.
2. Remove all APIs that allow passing individual data sources to process. Only keep the methods that pass groups in for processing.