Return a b-spline representing the antiderivative.
nu : int, optional
Antiderivative order. Default is 1.
b : BSpline object
A new instance representing the antiderivative.
If antiderivative is computed and
self.extrapolate='periodic', it will be set to False for the returned instance. This is done because the antiderivative is no longer periodic and its correct evaluation outside of the initially given x interval is difficult.