Remove all data points that belong to spectra that did not retain at
        least **percAccept** percent of the number of data points.

        ..warning::

            This function does not honor additional dimensions (e.g.,
            timesteps) yet!

def filter_incomplete_spectra(self, flimit=1000, percAccept=85):
        """Remove all data points that belong to spectra that did not retain at
        least **percAccept** percent of the number of data points.

        ..warning::

            This function does not honor additional dimensions (e.g.,
            timesteps) yet!

        """
        assert percAccept > 0 and percAccept < 100

        def _retain_only_complete_spectra(item, fmax, acceptN):
            """Function called using pd.filter, applied to all spectra in the
            data set. Return true if the number of data points <= **fmax** in
            item is equal, or larger, than **acceptN**.

            Parameters
            ----------
            item : :py:class:`pandas.DataFrame`
                dataframe containing one spectrum
            fmax : float
                maximum frequency up to which data points are counted
            acceptN : int
                the number of data points required to pass this test

            Returns
            -------
            true : bool
                if enough data points are present
            false : bool
                if not enough data points are present
            """
            frequencies = item['frequency'].loc[item['frequency'] < fmax]
            fN = frequencies.size
            if fN >= acceptN:
                return True
            return False

        group_abmn = self.data.groupby(['a', 'b', 'm', 'n'])
        frequencies = np.array(
            list(sorted(self.data.groupby('frequency').groups.keys()))
        )
        assert flimit >= frequencies.min() and flimit <= frequencies.max()
        Nlimit = len(np.where(frequencies <= flimit)[0])
        Naccept = np.ceil(Nlimit * percAccept / 100.0)
        self.data = group_abmn.filter(
            _retain_only_complete_spectra, fmax=flimit, acceptN=Naccept
        ).copy()