Some time in the past, the simple fixed momentum setting for particle gun was lost.

Now we set a fixed energy if the Energy property is set, otherwise we
use momentumMin and momentumMax to chose a momentum from a range.
Also changed some printouts to show the range we chose and the final
value used during simulation.

this worked accidentally in python2, but now the dictionary order is fixed and isotrop was set to false when parsing the default values, because it happens after the setting of distribution now

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

Add control over distributions
if a distribution is set activate isotrop
if isotrop is activated use uniform by default

Move set up gun function to Helper/Gun, keeps functionality closer together

If used isotropic is mandatory so we turn it on in  setup
Note different capitalisations for options,
keeping lower case starting char for ddsim 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