mincbbox: wrong step extent for non 1.0 step sizes

[gdevenyi]

Another issue is that it returns by default the wrong extent if the step
is not 1mm:

if (two_lines)
print ("%f %f %f\n%f %f %f\n",minx, miny, minz, maxx-minx+1, maxy-miny+1,
maxz-minz+1);
else
print ("%f %f %f %f %f %f\n",minx, miny, minz, maxx-minx+1, maxy-miny+1,
maxz-minz+1);

"max{x,y,z}-min{x,y,z}+1" should probably be replaced by
"max{x,y,z}-min{x,y,z}+step{x,y,z}" to report the extent of the voxel
dimensions (edge to edge) in world coordinates as I think is intended or
simply "max{x,y,z}-min{x,y,z}" to report the extent between the voxel
centers.

Reported on mailing list:
http://www.bic.mni.mcgill.ca/pipermail/minc-users/2017-January/004589.html