<tt><b>shearrate</b></tt> is a 3x3 matrix.  When <tt>applyshear = 1</tt>, 
then at every time step the simulation box matrix <tt><b>H</b></tt> is updated by,

<p>
<tt> <b>H</b> += <b>H</b> * <b>shearrate</b> * timestep </tt>.

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