Support conversion of Delphes output format events to edm4hep

- To support existing analyses using standalone Delphes implementations switching to the official approach using EDM4hep it would be extremely helpful to provide an additional exectuable to simply convert Delphes output format -> EDM4hep. Currently the fast simulation step has to be fully rerun in these cases (and depending on the setup, even the event generation).
- This could work by adapating the ROOT reader to parse everything, and running with an "empty" placeholder Delphes card that only defines the TreeWriter (based on what is in the input file).

The issue names no files or tests; it proposes adapting the ROOT reader and using an empty Delphes card with a TreeWriter. Start by locating the reader and existing executable entry points, then determine how input events and EDM4hep output are represented. Done means an executable can convert existing Delphes output without rerunning fast simulation.

Written by the indexing model from the issue text.