Response supports two abstract subtypes: RealResponse, which implements detector responses comprised of a single RMF (the normal case) and MultiResponse, which allows for Responses to be composed of linear
combinations of RMFs. MultiResponse is a new facility, which supports the Integral/SPI instrument, but may well be useful in future missions. As is the case for DataSet, the Response classes are made concrete by
inheriting from a separate IO interface class. The third, a concrete subtype, is DummyResponses which are generated by the user for testing and plotting purposes.