<li><id> = id to assign to the collection of FE faces</li>
<li><xmin> <xmax> <ymin> <ymax> <zmin> <zmax> = coordinates of the bounding box that is coincident with the desired FE faces</li>
<li><in|out> = "in" gives inner faces to the box, "out" gives the outer faces to the box</li>
<li>units = option to specify real as opposed to lattice units </li>