code: private IIsotope getIsotope(IIsotope isotope) {
          for (IIsotope thisIsotope : isotopes()) {
              if (isTheSame(isotope, thisIsotope)) return thisIsotope;
          }
          return null;
      }
docs: Get the isotope instance given an IIsotope. The instance is those
  that has the isotope with the same symbol, natural abundance and
  exact mass.
  
  @param  isotope The IIsotope for looking for
  @return         The IIsotope instance
  @see            #isotopes