export a graph to CSV for simpler viewing

def export(g, csv_fname):
    """ export a graph to CSV for simpler viewing """
    with open(csv_fname, "w") as f:
        num_tuples = 0
        f.write('"num","subject","predicate","object"\n')
        for subj, pred, obj in g:
            num_tuples += 1
            f.write('"' + str(num_tuples) + '",')
            f.write('"' + get_string_from_rdf(subj) + '",')
            f.write('"' + get_string_from_rdf(pred) + '",')
            f.write('"' + get_string_from_rdf(obj) + '"\n')
    print("Finished exporting " , num_tuples, " tuples")