BUG: Made grid transform writing more robust

Float and double grids can be saved (earlier floats were not supported and Slicer could crash when attempted)
To/From parent transform can be saved as well (earlier inverse transform saving was not working when the inverse transform had never been used before)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24555 3bd1e089-480b-0410-8dfb-8563597acbee