code: @Override
      public Double getExactMass() {
          double totalMass = 0.0;
          for (IAtom atom : fragment.atoms()) {
              totalMass += atom.getExactMass();
          }
          return totalMass;
      }
docs: The exact mass of an FragmentAtom is defined as the sum of exact masses
  of the IAtom's in the fragment.