NumCosmoXCOR_KERNEL_MIN_USEFUL_PEAK_EPSILON

Smallest NcXcorKernel:peak-epsilon worth asking for. The tolerance is a fraction of the peak of $W_i(k)$, but the quantity integrated to form $C_\ell$ is $k^2 W_i W_j$, so it enters squared: this floor is $10^{-12}$ on the integrand, already past what the outer $k$ integral carries. Below it nc_xcor_kernel_set_peak_epsilon() warns.