DDSim: add option to set userInputPlugin for simulation

[andresailer]

BEGINRELEASENOTES

• DDSim: add option to set userInputPlugin for simulation by adding a plugin for themselves, and this to the ddsim steering file (for Integration of external library (CRY) for cosmic ray generation in DDG4 simulation (Question) #940)

     def exampleUserPlugin(dd4hepSimulation):
       '''Example code for user created plugin.
  
       :param DD4hepSimulation dd4hepSimulation: The DD4hepSimulation instance, so all parameters can be accessed
       :return: GeneratorAction
       '''
       from DDG4 import GeneratorAction, Kernel
       # Geant4InputAction is the type of plugin, Cry1 just an identifier
       gen = GeneratorAction(Kernel(), 'Geant4InputAction/Cry1' , True)
       # CRYEventReader is the actual plugin, steeringFile its constructor parameter
       gen.Input = 'CRYEventReader|' + 'steeringFile'
       # we can give a dictionary of Parameters that has to be interpreted by the setParameters function of the plugin
       gen.Parameters = {'DataFilePath': '/path/to/files/data'}
       gen.enableUI()
       return gen
  
     SIM.inputConfig.userInputPlugin = exampleUserPlugin

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