code: public static IMolecularFormula getMolecularFormula(String stringMF, IMolecularFormula formula) {
          return getMolecularFormula(stringMF, formula, false);
      }
docs: add in a instance of IMolecularFormula the elements extracts form
  molecular formula string. The string is immediately analyzed and a set of Nodes
  is built based on this analysis
  <p> The hydrogens must be implicit.
  
  @param  stringMF   The molecularFormula string
  @return            The filled IMolecularFormula
  @see               #getMolecularFormula(String, IChemObjectBuilder)