FIX: Fixed volume rendering related memory leak.

Compare View Link: https://github.com/paulcm/LongitudinalPETCT/compare/7c06d40...7f76f6e

Fixed memory leak caused by incorrect volume rendering node creation in Python after reading:
http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Modules/VolumeRendering#How_Tos