A 3x3 matrix specifying the external stress (in MPa) applied to the simulation cell.  This
affects <tt>relax</tt> if <tt>conj_fixbox = 0</tt>, as well as <tt>run</tt> if
<tt>ensemble_type = NPT</tt> or <tt>NPH</tt>.

<p>
The sign convention of MD++ is such that a hydrostatic pressure corresponds to
a positive value in stress, while tensile loading corresponds to a negative value.

<p>
We need to make sure <tt>stress</tt> is set to a symmetric matrix.
