Multithreaded Pythia8 interface

Copied and refactored Pythia8Production into Pythia8ProductionMT which holds an instance and thread-local Pythia8 instance and can thus run lock free. Pythia instances are created on demand when a thread+instance combination first tries to generate an event. Calls to all configuration functions done e.g. from ExternalGenerator are remember and applied during each of those per thread initialization.

Target: sim prototype branch for G4MT interface performance studies