start with the least significant bit
     (the 'lcio' convention)
   - unused hight bits are still all 1
     this is 'cured' in BitField64.setValue() 

   => ideally we should try an use 0 as default value
      for the IDDescriptors and unused bits
      this would allow to directly use the 
      VolumeID of the sensitive volume as cellID
      for the hits
   ( I did not manage to get this working, though.)