BUG: remove node SceneRootDir, use RootDirectory on MRML scene

The node ivar scene root directory wasn't staying updated when the root directory was changing on the scene. Updated the code to refer to the scene for the root directory.
One important change is that in the XML parser, the scene has to be set on a storage node before reading/writing it so that the absolute path can be determined.
Also set the scene root dir more consistently
Updated the fiber bundle node test to use the testing macros since it was reproducing code.

Issue 0003956

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24393 3bd1e089-480b-0410-8dfb-8563597acbee