mincbbox: hard coded step size 1.0

[gdevenyi]

1. The first issue is with the option -mincresample, where it will print a
   step of 1.0 regardless of the actual step of the volume:

print ("-step 1.0 1.0 1.0 -start %f %f %f -nelements %d %d %d\n",
minx, miny, minz, VIO_ROUND(maxx-minx)+1, VIO_ROUND(maxy-miny)+1,
VIO_ROUND(maxz-minz)+1);

The step should probably be
(max{x,y,z}-min{x,y,z})/(v_max{x,y,z}-v_min{x,y,z}).

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