Mean of non-NA/null values

        Returns
        -------
        mean : float

def mean(self, axis=0, *args, **kwargs):
        """
        Mean of non-NA/null values

        Returns
        -------
        mean : float
        """
        nv.validate_mean(args, kwargs)
        valid_vals = self._valid_sp_values
        sp_sum = valid_vals.sum()
        ct = len(valid_vals)

        if self._null_fill_value:
            return sp_sum / ct
        else:
            nsparse = self.sp_index.ngaps
            return (sp_sum + self.fill_value * nsparse) / (ct + nsparse)