Enable filtering of labelmaps to show only one label.

Example:

var v = new X.volume();
v.file = 'test.nrrd';
v.labelmap.file = 'seg.nrrd';
v.labelmap.colortable.file = 'colors.txt';

v.labelmap.showOnly = 3; // show only label value 3
// or
v.labelmap.showOnly = [1,0,0,1]; // show only the fully red label