BUG: Fix vtkObserverManager warning happening after loading a file.

This commit ensured that the observer associated with MRMLNodesCallbackCommand
are removed also if the file is successfully loaded.

Warning: In /home/jchris/Projects/Slicer/Libs/MRML/Core/vtkObserverManager.cxx, line 59
vtkObserverManager (0x3b99ce0): The callback is not deleted because there
are still some observation. They seem to not have been registered into
the event broker.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22569 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Prevent leaks when factory methods are used from python

There is currently no automatic/clean mechanism to release the object
created by factory methods.

The only "hack" that exists for now is to decrease the reference
count manually in your code:

For more details, see [1], [2], [3] and [4]

[1] http://wiki.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/MemoryManagement
[2] http://massmail.spl.harvard.edu/public-archives/slicer-devel/2011/007513.html
[3] http://www.vtk.org/Bug/view.php?id=12139
[4] http://www.vtk.org/pipermail/vtk-developers/2011-April/009757.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22570 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fix memory leaks in vtkMRMLModelSliceDisplayableManager

See inlined comment for details.

This commit will also avoid leaks (and by extension ensure they
pass when leaks detection is enabled) in the following tests:
  py_NeurosurgicalPlanningTutorial
  py_ThresholdThreadingTest
  py_NeurosurgicalPlanningTutorial
  py_ScenePerformance

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22571 3bd1e089-480b-0410-8dfb-8563597acbee