New version from Andrei and Mihaela:

 - several implementations of method Safety for shapes
 - few bug fixes in safety computation (phi segmented shapes)
 - volumes have now an unique ID that you can get with GetNumber()
 - Mihaela implemented an overlap checker that can be called either for
all geometry :
    TGeoManager::CheckOverlaps(Double_t ovlp=0.1) or just for a given
volume. OVLP is the lower overlapping limit. The checker looks for the
following errors in the geometry definition :
  1 - nodes extruding their mother - can be visualised with
TGeoManager::DrawExtrusion(char *mother, char *node) which draws mother
in blue and extruding daughter in red
  2 - ONLY brothers overlapping to each other - can be  visualized with
TGeoManager::DrawOverlap(char *volume, char *node1, char *node2) - node1
will appear in red and node2 in blue.
  These methods can be called only after CheckOverlaps() that prints the
list of overlaps. They are all in the context menu of both TGeoVolume
and TGeoManager.


git-svn-id: http://root.cern.ch/svn/root/trunk@6068 27541ba8-7e3a-0410-8455-c3a389f83636