Returns new histogram with all data cumulated along axis.

def cumulate(self, axis):
        """Returns new histogram with all data cumulated along axis."""
        axis = self.get_axis_number(axis)
        return Histdd.from_histogram(np.cumsum(self.histogram, axis=axis),
                                     bin_edges=self.bin_edges,
                                     axis_names=self.axis_names)