Add rigid body rotation velocity field to the simulation cell so that
its total angular momentum around a given axis is zero.  The axis is
specified by <tt>zerorot</tt> which can be <tt>"x"</tt>, or <tt>"y"</tt>, 
or <tt>"z"</tt>.

<p>
There is usually no need to call this command manually.
<tt>initvelocity</tt> is supposed to accomplish that after randomizing the velocities.

<p>
Also see <tt>L_com</tt>.
