DDSim: Add logic to add help for non-property helper variables, add logic to add choices for all helper options

Move module doc to class doc so we can use it in the automatically created steering file

Change docstrings to be more sensible

* 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