Mark all atoms <tt>i</tt> whose <tt>fixed[i] == 1</tt> as removed, i.e. set <tt>fixed[i] = -1</tt>.
An atom <tt>i</tt> with <tt>fixed[i] == -1</tt> will be ignored in potential function / force evaluations.
So they are effectively removed, but can be quickly restored without changing the indices of other
atoms.

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