Each CPU marks atoms <tt><i>i</i></tt> outside the domain boundary (by more than a skin distance)
as <tt>fixed[<i>i</i>] = -1</tt>.

<p>
Effective only when MD++ is compiled for parallel runs, e.g. with
<tt>SYS = mpicc</tt> or <tt>SYS = mpigpp</tt>.

