EDProducer to read data products from another event data file.
This time for sure ... The use case is a job that runs a signal event generator, passes it through G4, reads in G4 output from many other files, each containing various backgrounds that have been processed through G4, then run the code that makes simulated hits from the ensemble of G4 outputs. The data products from the many files would be distinguished by having different process names. In the initial release we do not need full tracking of provenance from the original files - it is sufficient for their provenance to be that of the EDProducer that read them in. We can talk later about what provenance info might or might not need to be imported from the parent files.