Set all atoms specified in the <tt>constrainatoms<tt> array to have be fixed
(i.e. <tt>fixed[i] = 1</tt>).

<p>
This is the inverse of the command <tt>constrain_fixedatoms</tt>.  This command
is useful to visualize which atoms are included in the <tt>constrainatoms</tt>
because fixed atoms are usually plotted in yellow.

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