Evaluate the potential energy change if a new particle is randomly inserted into the simulation cell.
Repeated calls to this function generate statistics that can be used to evaluate the chemical
potential.  Since the number of atoms will need to increase, make sure to
set <tt>allocmultiple</tt> to be greater than 1 when calling <tt>makecrystal</tt> or 
<tt>readcn</tt> above.

<p>
Reference: Frenkel and Smit, <i>Understanding Molecular Simulation</i>, 2nd ed., Section 7.2, 
Chemical Potentials.

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