Save the beam conditions to a metadata tree

The beam energies and divergence settings used in the simulation should be stored so they can be read back by EICrecon. This is essential for correct reconstruction of the FF/FB particles.

Currently there are metadata trees written by podio to the edm4hep file but it's not clear to me if DD4hep has an interface to those or whether this needs to be implemented there before we can add it into the epic workflow.