BUG: fix backward compatibility of sample data logic methods

Since the new generic API returns lists of loaded nodes, the
backward compatibility methods need to return the first element
rather than the list.

py_ThresholdThreadingTest
py_StandaloneEditorWidgetTest
py_sceneImport2428
py_ViewControllersSliceInterpolationBug1926

http://slicer.cdash.org/testDetails.php?test=4166721&build=129681
http://slicer.cdash.org/testDetails.php?test=4166722&build=129681
http://slicer.cdash.org/testDetails.php?test=4166791&build=129681
http://slicer.cdash.org/testDetails.php?test=4166797&build=129681

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22225 3bd1e089-480b-0410-8dfb-8563597acbee