Refactor and improve features for ddsim

* Move DD4hepSimulation et al into python module DDSim, needed for proper
  importing

* Can now control magneticfield setup, gun, particle handler, output via command
  line.

** added ConfigHelper and daughter classes implementing these options
   DDSim/DDSim/Helper, easily extensible to additional features

** also easier changing of these parameters via steering file See
   example/steeringFile.py

*** No more functions, no more "global GeV"

** --help string can be given by using @property, see e.g. Helper/Output.py

* added optional argcomplete, allows command line "tab"complete for all command line options (and values!)
  Needs argcomplete (sudo pip install argcomplete), bash completion and bash (or zsh).
  and 'eval "$(register-python-argcomplete ddsim)"'

* move import statements to reduce startup time until "--help", mandatory to
  reduce latency for argcomplete