BUG: Fix VolumeProperty file reading with gcc < 4.2.2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35397
Closes #3220

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22167 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Make volume property file reading more robust

* nextHigher does no longer produce denormalized values, it's error prone
* ensure values are are unique when reading from file
Issue #3220

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22168 3bd1e089-480b-0410-8dfb-8563597acbee