This routine computes the \(n\)-th normalized hydrogenic bound state
radial wavefunction,
\[R_n := 2 (Z^{3/2}/n^2) \sqrt{(n-l-1)!/(n+l)!} \exp(-Z r/n) (2Zr/n)^l
L^{2l+1}_{n-l-1}(2Zr/n).\]
where \(L^a_b(x)\) is the generalized Laguerre polynomial
(see Laguerre Functions). The normalization is chosen such that
the wavefunction \(\psi\) is given by \(\psi(n,l,r) = R_n Y_{lm}\).