Current step number in MD simulation <tt>run</tt>.

<p>
It is used to determine when whether the X-window needs to be 
refreshed, or intermediate .cn file needs to be saved, at the current 
simulation step.

<p>
Also see <tt>plotfreq</tt>, <tt>savecnfreq</tt>, <tt>continue_curstep</tt>, <tt>conj_step</tt>.
