BUG: Fix crash when loading volume

Some compilers (e.g. i686-apple-darwin10-g++-4.2.1) fail when using RTTI
with vtkDataArrayTemplate.
VTKv6 offers the FastDownCast workaround by internally using static_cast.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23133 3bd1e089-480b-0410-8dfb-8563597acbee