A string specifying which component(s) of the angular momentum need to be zeroed out
in <tt>initvelocity</tt>.

<p>
For example, for a nanowire under PBC along z axis but subjected to
free surface boundary conditions in x and y directions, we need to set <tt>zerorot = z</tt>.
For a nanoparticle under free surface boundary conditions in x, y, and z directions, we
need to set <tt>zerorot = all</tt>.
