fix comparison function in presence of None values

            for col_pair in other_col_pairs:

                col_prev = col_pair[0]

                col_curr = col_pair[1]

                try:

                    diff_other += abs(float(x[col_curr]) - float(x[col_prev]))

                except ValueError:

                    # can't make float from the column contents, probably NONE -> put to top

                    diff_other += float("inf")

            if diff_other > 0:

                diff = -1000000