Add two cmake functions for creating dd4hep plugins (creating library and rootmaps and installing package env file)

   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

Change options _WITH_ to _USE_
Print Warning that variables with _WITH_ are deprecated
Change Defintions to _USE_ as well
Add REQUIRED to find_package geant4 and xercesc if they are turned on
Updated documentation
Updated ILDExDet example

Export writes information in ~/.cmake/packages, making it hard for cmake not to find ddsegmentation (even when it is removed)

Rename DDSegmentation_INCLUDE_DIR to ..._DIRS
Fix standalone build of DDSegmentation, set DDSegmentation_LIBRARIES in DDSegmentationConfig.cmake
Keep the choice to use external DDSegmentation, set _LIBRARIES and INCLUDE_DIRS in DDSegmentation CMakeLists.txt in case of internal build

   in order to keep simple build sequence for DD4hep
   

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