NumCosmoMathSphereXito_chebyshev

Computes, for every row of C_ells, the Chebyshev coefficients $c_k$ of the angular correlation function truncated at $\ell_\mathrm{max}$, $$ \xi(\theta) = \sum_{\ell=0}^{\ell_\mathrm{max}} \frac{2\ell + 1}{4\pi} C_\ell P_\ell(\cos\theta) = \sum_{k=0}^{\ell_\mathrm{max}} c_k T_k(\cos\theta) = \sum_{k=0}^{\ell_\mathrm{max}} c_k \cos(k\theta), $$ the multipoles below lmin taken as zero. The cost is $\ell_\mathrm{max}^2 / 4$ multiplications per spectrum. Evaluate the result with ncm_sphere_xi_eval_chebyshev() or ncm_sphere_xi_eval_chebyshev_grid(), which work in $\theta$; a Clenshaw sum in $x = \cos\theta$, such as ncm_spectral_chebyshev_eval(), forms $x$ and loses accuracy at small $\theta$: measured $2$ to $4 \times 10^{-11}$ of the largest $|\xi|$ on $[2.5’, 250’]$ for $\ell_\mathrm{max}$ from 2000 to $10^5$. Aborts when NcmSphereXi:lmax is zero, when the last column of C_ells is above NcmSphereXi:lmax, and when c does not have the rows of C_ells and $\ell_\mathrm{max} + 1$ columns. The scratch space belongs to sxi, so concurrent calls on one object are not allowed.