Non destructive category removal.

        Parameters
        ----------
        categories : list
            list of categories to keep
        ignore_absences : bool, False by default
            if categories does not appear, don't raise an error, just move on.

        Returns
        -------
        TermDocMatrix, new object with categories removed.

def keep_only_these_categories(self, categories, ignore_absences=False):
        '''
        Non destructive category removal.

        Parameters
        ----------
        categories : list
            list of categories to keep
        ignore_absences : bool, False by default
            if categories does not appear, don't raise an error, just move on.

        Returns
        -------
        TermDocMatrix, new object with categories removed.
        '''
        if not ignore_absences:
            assert set(self.get_categories()) & set(categories) == set(categories)
        categories_to_remove = [c for c in self.get_categories() if c not in categories]
        return self.remove_categories(categories_to_remove)