Number of groups of atoms that are treated separately in the REBOLJ potential.

<p>
Atoms within the same <tt>group</tt> interact with the REBO potential.  Atoms
in different <tt>group</tt> interact with the LJ potential.  The atomic configuration
need to have the <tt>group</tt> array (in addition to the <tt>species</tt> array, if
both Carbon and Hydrogen are present) set up accordingly for this to work properly.

