Create an elliptic atomically sharp crack by removing the interaction between two
sides of a cut-plane.  All atomic pairs that reside on two sides of the cut plane
are registered in the <tt>nl_skip_pairs</tt> array, so that they will not appear 
on each other's neighbor list, effectively removing their interactions.

<p>
Usage: <tt>input = [ <i> enable zind  yind  sx0 sy0 sy1 sz0 sz1 </i> ]</tt>

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