This class must implement the following method:

    /**
     * Determine if two atoms are isotopes of the
     * same Element (i.e., have the same number of protons
     * but different numbers of neutrons)
     *
     * @param  a   One Atom
     * @param  b   The other Atom
     * @return     true if a and b are isotopes
     */
    public static boolean areIsotopes(Atom a, Atom b)
    {
    }