Fixed round-off issue in ROI3DStack.contains()

writing ```(int) z``` poses issues when checking lower bounds.
For instance, let us assume the ROI3D starts at z=0. Here a call to ```contains(anyX, anyY, -0.1)``` will return true because -0.1 is rounded off to 0, while it should return false.
Merge pull request #613 from adufour/patch-3

Fixed round-off issue in ROI3DStack.contains()