pythonPackages.geant4: init at 10.6.2

Starting geant4 10.6.2 g4py can not be built separately
http://geant4-data.web.cern.ch/geant4-data/ReleaseNotes/Patch4.10.6-2.txt

Also, it appears that g4py itself is now deprecated, it was moved
to environments/g4py/tests/g4pytest in the source distribution.  The only
remaining imported module is Geant4, hence python package name
`pythonPackages.geant4`, the capitalization matches the one of the non-python
attribute.

pkgs/top-level/python-packages.nix

@@ -3886,6 +3886,11 @@ in {
 
   gdrivefs = callPackage ../development/python-modules/gdrivefs { };
 
+  geant4 = disabledIf (!isPy3k) (toPythonModule (pkgs.geant4.override {
+    enablePython = true;
+    python3 = python;
+  }));
+
   genshi = callPackage ../development/python-modules/genshi { };
 
   gentools = callPackage ../development/python-modules/gentools { };