For debugging purposes.  <tt>output_str</tt> is the string that contains the numerical values 
of the variables specified in <tt>output_fmt</tt>.  During MD simulation (<tt>run</tt>), every
<tt>savepropfreq</tt> steps the <tt>output_str</tt> string is evaluated and gets written
into the property file.

<p>
Also see <tt>saveprop</tt>, <tt>outpropfile</tt>.
