Function spfunc::gamma::polygamma

pub fn polygamma<T: FromComplex>(z: T, n: usize) -> T

Calculate the polygamma function $\Psi^{(n)}(z)$, which is defined as the n-th derivative of the digamma function.

$$\Psi^{(n)}(z)\equiv\frac{d^{n+1}}{dz^{n+1}}\ln{\Gamma(z)}=\frac{d^{n}}{dz^{n}}\Psi(z)$$

When n = 0, it is same as the digamma function $\Psi(z)$.