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

   such that other user actions can access it (e.g. for adding the MCParticle collection)

    - renamed namespace tgeo to dd4hep
       -> users need to replace tgeo::mm by dd4hep::mm

Hopefully I got the assemblies right now. Assembly test added in examples/ClientTests. Please see also doc/release.notes