Maximum number of neighbors for each atom in the plot-neighbor-list, i.e. the neighbor list
used for plotting and geometric analysis purposes.  If this neighbor list is constructed for
a reference configuration, and is not updated when a newer configuration is loaded, it allows
us to compute the change of bond lengths between atoms and their original neighbors.

<p>
Also see <tt>construct_plot_nnlist</tt>, <tt>caldisregistry</tt>.
