DD4Hep integration

[ggalgoczi]

Since epic stack uses DD4Hep we need to integrate eic-opticks into it

[ggalgoczi]

We need access to SteppingAction, EndofEventAction and EndOfRunAction. I wrote "Actor" classes for the opengate integration. Here we could use the same philosophy according to a brief discussion with @rahmans1 . They are called "Plugins" here.

Example plugins: https://github.com/eic/npsim/tree/main/src/plugins/include/npdet

[ggalgoczi]

Working plugins for the Celeritas project: https://github.com/celeritas-project/celeritas/tree/develop/src/ddceler

[ggalgoczi]

Pushed first version of plugins into https://github.com/BNLNPPS/eic-opticks/tree/dd4hep_integration