fn legendre_factor_pair<const N: usize>( l: i32, m: i32, z: Jet<N>, ) -> [Jet<N>; 2]
F_l^m and F_(l-1)^m of legendre_factors, with F_(l-1)^m = 0 for l = |m| and both zero for |m| > l.
F_l^m
F_(l-1)^m
legendre_factors
F_(l-1)^m = 0
l = |m|
|m| > l