Probably resolve WB-657: Better support of volume files with oblique sform: If...

Probably resolve WB-657: Better support of volume files with oblique sform: If an oblique volume is selected as the underlay, the viewing orientation will switch to oblique and orthogonal is removed from the menu.  In addtion, when a slice index was changed while viewing an oblique volume, the slices changed to dramatically different values.  Now, incrementing or decrementing the slice indices will adjust the corresponding coordinate by the size of voxel.