Documentation update and minor fix to refine

        method : object

                 A function that accepts and ndarray and returns an object

                 with a bins attribute

        df      : dataframe

                Dataframe (from which a ndarray will be extracted) to pass to the method.

        bin_id : int

                 The index into the bins object.  Data classified > this

                 id is masked

        fj = method(df.values.ravel(), **kwargs)

        bins = fj.bins

        # Mask the data that falls outside the provided bins

        mask = df <= bins[bin_id]

        mask = df.iloc[:, 0] <= bins[bin_id]

        new_x1 = self.x1.iloc[mask[mask == True].index]

        new_x2 = self.x2.iloc[mask[mask == True].index]

        fmatrix, new_mask = compute_fundamental_matrix(new_x1.values, new_x2.values)