Bug Fix: Oblique volume drawing was not working for a volume file that...

Bug Fix: Oblique volume drawing was not working for a volume file that contained a single slice (K-dim => 1).  Problem was traced to VolumeFile::interpolateValue().  CUBIC and TRILINEAR interpolation modes require two slices to function since they interpolate between two slices.   Now VolumeFile::validateMembers() examines the dimensions and will set a flag 'm_singleSliceFlag' if any of the dimensions are one.  VolumeFile::interpolateValue() will override the interpolate mode parameter to ENCLOSING_VOXEL if the 'm_singleSliceFlag' is set.