Specify the potential model used in MDCASK. Choices are
<tt>SW</tt>, <tt>TERSOFF</tt>, <tt>ROCKETT</tt>, <tt>ZBL</tt>,
<tt>EAM</tt>, <tt>EDIP</tt>, <tt>MEAM</tt>.  
It needs to be set before calling <tt>readMDCASK</tt>.
