Fill NaN values by propogating values backward

        *Requires bottleneck.*

        Parameters
        ----------
        dim : str
            Specifies the dimension along which to propagate values when
            filling.
        limit : int, default None
            The maximum number of consecutive NaN values to backward fill. In
            other words, if there is a gap with more than this number of
            consecutive NaNs, it will only be partially filled. Must be greater
            than 0 or None for no limit.

        Returns
        -------
        Dataset

def bfill(self, dim, limit=None):
        '''Fill NaN values by propogating values backward

        *Requires bottleneck.*

        Parameters
        ----------
        dim : str
            Specifies the dimension along which to propagate values when
            filling.
        limit : int, default None
            The maximum number of consecutive NaN values to backward fill. In
            other words, if there is a gap with more than this number of
            consecutive NaNs, it will only be partially filled. Must be greater
            than 0 or None for no limit.

        Returns
        -------
        Dataset
        '''
        from .missing import bfill, _apply_over_vars_with_dim

        new = _apply_over_vars_with_dim(bfill, self, dim=dim, limit=limit)
        return new