Added SqrtFilter to get difference in HU not HU2
Added new level and window values for SquaredDifference
Removed Checkerboard and SquaredDifference Volumes - for now they remain in Node, will remove them, if everything works.
Added defult Enabled=False for all buttons. And logic to Enabled=True, when all apropriate volumes are loaded.
Changed checkerboard pattern to int from string (added slider to choose value)