ENH: stress test saving and restoring MRBs, util methods added

Add a test to save and restore MRBs in a loop, fixing the bug note in the
save restore test as well.
Clean up the testing of saving and loading MRBs by exposing the core io
manager loadScene method to Python, and adding a saveScene one. This will
simplify testing since it uses detects that it needs to use the scene writer
which will use the application logic save to MRB bundle directory + zip calls,
so from python you won't need to make the two calls with attendant managing of
temporary directories. Added a note to the application logic header file
pointing to saveScene for general use.

Issue 0003956

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24390 3bd1e089-480b-0410-8dfb-8563597acbee