If <tt>saveprop = 1</tt> and <tt>savepropfreq != 0</tt>, then during MD simulation
(invoked by the <tt>run</tt> command), a line is written in the property file
(named by variable <tt>outpropfile</tt>) every <tt>savepropfreq</tt> steps. The
content of the property file is specified by the <tt>outputfmt</tt> variable.
