write it as a collection of individual sparse series

def write(self, obj, **kwargs):
        """ write it as a collection of individual sparse series """
        super().write(obj, **kwargs)
        for name, ss in obj.items():
            key = 'sparse_series_{name}'.format(name=name)
            if key not in self.group._v_children:
                node = self._handle.create_group(self.group, key)
            else:
                node = getattr(self.group, key)
            s = SparseSeriesFixed(self.parent, node)
            s.write(ss)
        self.attrs.default_fill_value = obj.default_fill_value
        self.attrs.default_kind = obj.default_kind
        self.write_index('columns', obj.columns)