code: public INode getNode(IAtom atom) {
  
          for (Map.Entry<INode, IAtom> v : nodeBondMap.entrySet()) {
              if (v.getValue().equals(atom)) {
                  return v.getKey();
              }
          }
  
          return null;
      }
docs: Return a node for a given atom else return null
  @param atom
  @return Node in the graph for a given atom