First implementation of dipole field, HepMC reader for DDG4 and fix to import constants into DDG4 python

   - don't write particles created in calo
     unless scattered back into tracker)
   - don't write particles created in tracker
     if below MinimalKineticEnergy ( Geant4ParticleHandler)

 - fixed generator status: set to 0 if
   particle created in simulation

small bug fix for older compilers/os by Frank Gaede: pow(x,x) function is ambiguous with implicit conversion

Improved version of MC particle handler, fix calorimeter hit aggregation with multiple contributions

Moved find_package calls to main cmakelists
moved include_directories to main cmakelists
only use dd4hep_generate_rootmap

Phi2 is the deltaPhi in Geant4, it is the EndPhi in TGeo

   part of DD4hep, i.e. no external library option
   - however DDSegementationConfig is created for other packages to 
     build against it
 - changed library versioning of DDSegmentation to be in synch w/ DD4hep
   ( versions and so-versions)
 - some cleanup of geant4 stuff