Add CMB lensing power spectrum

[hersle]

Work in progress to fix #35.

using SymBoltz, Plots
M = ΛCDM(h = nothing, ν = nothing, K = nothing)
pars = parameters_Planck18(M)
prob = CosmologyProblem(M, pars)

ls = 10:10:1000
Cls = spectrum_cmb(:ψψ, prob, ls; Nlos = 8196)

plot(@. log10(ls), @. log10(ls^2 * (ls+1)^2 / 2π * Cls))