TVirtualMC
- Added new function TVirtualMC::MediumId(const const Text_t* mediumName)
- Removed default implementations for the recent functions
  and made them pure anstract (=0)
- Removed deprecated function GetMedium() replaced with CurrentMedium()
- Copy constructors and assignment operators moved to private and
  their implementation is removed

TVirtualMCGeometry
TGeoMCGeometry
- Added new function TVirtualMC::MediumId(const const Text_t* mediumName)
- Removed singleton pattern (now more than one instance may exist)
- Copy constructors and assignment operators moved to private and
  their implementation is removed


git-svn-id: http://root.cern.ch/svn/root/trunk@17193 27541ba8-7e3a-0410-8455-c3a389f83636