Remove missing values according to Baloo's convention.

        Parameters
        ----------
        subset : list of str, optional
            Which columns to check for missing values in.

        Returns
        -------
        DataFrame
            DataFrame with no null values in columns.

def dropna(self, subset=None):
        """Remove missing values according to Baloo's convention.

        Parameters
        ----------
        subset : list of str, optional
            Which columns to check for missing values in.

        Returns
        -------
        DataFrame
            DataFrame with no null values in columns.

        """
        subset = check_and_obtain_subset_columns(subset, self)
        not_nas = [v.notna() for v in self[subset]._iter()]
        and_filter = reduce(lambda x, y: x & y, not_nas)

        return self[and_filter]