Removed the 'custom node group' example from the pynodes template script. This does not work properly due to the fact that node groups don't have a single registerable base class any more. The reason for
that is that RNA does not support multiple inheritance so the actual node group subtypes (ShaderNodeGroup, CompositorNodeGroup, TextureNodeGroup) can not be derived from both the
ShaderNode/CompositorNode/TextureNode base types as well as a common NodeGroup type ... It is possible however to define node group types entirely in python which avoids the limitations of the RNA system
and is much more flexible, example for this will follow later.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57300 954f8c5b-7b00-dc11-b283-0030488c597c
Added an example for using the new node categories system to the custom_nodes.py template script.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57301 954f8c5b-7b00-dc11-b283-0030488c597c