Usage: <tt>input = <i>id</i> setfixedatomsgroup</tt>

<p>
Set <tt>group[i] = <i>id</i></tt> for all atoms <tt>i</tt> whose <tt>fixed[i] == 1</tt>.

<p>
Also see <tt>fixatoms_by_pos_topol</tt>, <tt>fixatoms_by_ID</tt>, <tt>fixatoms_by_group</tt>.
