BUG: Prevent ITK from crashing after reading TPS transform

ITK crashes if the loaded transform contains TPS transform, as it
requires manual initialization (calling of ComputeWMatrix).

This error was reported in 2011 (http://public.kitware.com/pipermail/insight-users/2011-September/042419.html)
but no action was taken by the ITK community. BRAINS toolkit applies a workaround to call ComputeWMatrix manually.
Therefore, we apply a similar workaround here.

Also changed default transform file extension to .h5 to allow passing composite transforms.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25690 3bd1e089-480b-0410-8dfb-8563597acbee