Fix wall/gran/region for granular models

These animations illustrate the fix wall/gran/region command, which lets a geometric region act as a boundary for granular particles. Regions can be unions or intersections of primitive shapes and can translate or rotate in time. The examples are a funnel, a rotating mixer, and a closed box. The first two were rendered with VMD; the last with the LAMMPS dump image command (which does not draw the enclosing box).