``zero lo voxels'' - zero out dim voxels

VFX name:

iLowZero.

IMPROPTU name:

Zero_lo_voxels.

purpose:

Zero out all voxels less than a user-preset gray-scale cutoff.

input:

An 8-bit volume.

output:

An 8-bit volume.

parameters:

Default parameters menu is

upper_threshold = 0

comments:

1.

A very fast function.