Wigner D-matrix elements for rotation group representations.
Differs from the .wl reference: `WignerD[{1,0,0}, Pi/2]`
WignerD[{j, m1, m2}, ψ, θ, ϕ]
WignerD[{j, m1, m2}, θ, ϕ]
WignerD[{j, m1, m2}, θ]
WignerD[{1, 0, 1}, ψ, θ, ϕ]
→ -(Sqrt[2]*E^(I*ϕ)*Cos[θ/2]*Sin[θ/2])WignerD[{1 / 2, 1 / 2, 1 / 2}, ψ, θ, ϕ]
→ E^(I/2*ψ + I/2*ϕ)*Cos[θ/2]WignerD[{2, 1, -1}, ψ, θ, ϕ]
→ 6*E^(I*ψ - I*ϕ)*((Cos[θ/2]^2*Sin[θ/2]^2)/2 - Sin[θ/2]^4/6)