Implement fast simulation shower modelling. Simplify GFlash to use the same sensitive actions as fast simulation

Implement fast simulation shower modelling. Simplify GFlash to use the same sensitive actions as fast simulation

emplace doesn't overwrite already existing collection, so this would lead to leaking memory

DDSim: compactFile can only be one instance, otherwise we cannot overwrite what is in the steeringfile, still takes multiple XML files

From https://stackoverflow.com/a/41153081

Needed for python2.7 compatibility

The XML files need to follow the structure where at the end is a closer file.
compactFile takes multiple arguments, or multiple instances of compactFile or both.

  ddsim --compactFile $DD4hep/examples/CLICSiD/compact/SiD_multiple_inputs.xml $DD4hep/examples/CLICSiD/compact/SiD_detectors_1.xml --compactFile $DD4hep/examples/CLICSiD/compact/SiD_detectors_2.xml $DD4hep/examples/CLICSiD/compact/SiD_close.xml --runType shell