Return a molecule which has largest graph in the compound
    Passing single molecule object will results as same as molutil.clone

def largest_graph(mol):
    """Return a molecule which has largest graph in the compound
    Passing single molecule object will results as same as molutil.clone
    """
    mol.require("Valence")
    mol.require("Topology")
    m = clone(mol)  # Avoid modification of original object
    if m.isolated:
        for k in itertools.chain.from_iterable(m.isolated):
            m.remove_atom(k)
    return m