When <tt>input = [ <i>g_id</i> <i>fx</i> <i>fy</i> <i>fz</i> ]</tt> is set before 
calling this command, then atoms with <tt>group[i] == <i>g_id</i></tt> will have
their <tt>Fext[i]</tt> vector set to <tt>(<i>fx</i>, <i>fy</i>, <i>fz</i>).  

<p>
If <tt>enable_Fext</tt> is set to 1, then <tt>Fext[i]</tt> will be added to the
force experienced by atom <tt>i</tt> for all atoms.
