TRestGeant4AnalysisProcess Geant4 processes upgrade

[jgalan]

I have seen the following code inside TRestGeantAnalysis::ProcessEvent.

    // process names as named by Geant4
    // processes present here will be added to the list of observables which can be used to see if the event
    // contains the process of interest.
    vector<string> processNames = {"phot", "compt"};
    for (const auto& processName : processNames) {
        Int_t containsProcess = 0;
        if (fOutputG4Event->ContainsProcess(fG4Metadata->GetGeant4PhysicsInfo().GetProcessID(processName))) {
            containsProcess = 1;
        }
        SetObservableValue("ContainsProcess" + processName, containsProcess);
    }

Perhaps, the processNames vector could be initialized through a metadata parameter. The metadata parameter could be just a string with comma separated values, and by default being "phot,compt". Then, the user may create any new process observables through this metadata parameter.