Add implementation for signal handling. Use generic signal handler in DDG4 for controlled shutdown on CTRL-C. Add test DDG4_SIGINT_handler in examples/DDG4

Add implementation for signal handling. Use generic signal handler in DDG4 for controlled shutdown on CTRL-C. Add test DDG4_SIGINT_handler in examples/DDG4

If we do not already store particles that become important'ish later we get a fatal error because the equilavent finding might not be able to find this particle

Allow to set MeanExcEnergy, MeanEnergyPerIonPair and BirksConstant in G4Materials ionisation parameters

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