Currently the _input_type property in every Transformer is interpreted as a specification of mandatory inputs. But there could conceivablye be cases where we want to allow for optional Stim types. We could potentially add an (optional) _optional_input_type field to supplement _input_type, which would afford some additional flexibility when writing Transformer classes. This is not a high priority since I can't think of any common use case we'd need it for right now, but I'm opening this anyway so I don't forget it.
adds support for optional input types; closes #105