Better automatic volume step for light microscopy

[Tom Goddard]

Most light microscopy 3d images have z voxel size 5 times larger than x and y. It is best to display these initially at step 2,2,1 or 4,4,1 if they are too large to show initially at full resolution. Current code will only adjust step to the same value in all 3 axes 2,2,2 or 4,4,4. If voxel size is anisotropic automatic adjustment should prefer setting step that gives more isotropic grid spacing, ie. keep step 1 along the z axis. Also the Volume Viewer panel should list step 2,2,1 and 4,4,1, ... in step menu to avoid having to type a command to get those.