Add static_assert for N_SLS > 1 and remove TODO comment

Co-authored-by: Rohit-Kakodkar <40695390+Rohit-Kakodkar@users.noreply.github.com>

Author: Copilot

core/specfem/attenuation/compute_tau_sigma.hpp

@@ -8,8 +8,6 @@
 namespace specfem {
 namespace attenuation {
 
-// TODO (Lucas : CPP20): and requires N_SLS>1
-
 /**
  * @brief Compute stress relaxation times \f$\tau_{\sigma}\f$ equally spaced in
  * log10 frequency.

core/specfem/attenuation/compute_tau_sigma.tpp

@@ -11,6 +11,7 @@ namespace attenuation {
 template <int N_SLS>
 Kokkos::View<type_real[N_SLS], Kokkos::LayoutRight, Kokkos::HostSpace>
 compute_tau_sigma(const type_real min_period, const type_real max_period) {
+  static_assert(N_SLS > 1, "N_SLS must be greater than 1 to avoid division by zero");
 
   Kokkos::View<type_real[N_SLS], Kokkos::LayoutRight, Kokkos::HostSpace> tau_s(
       "tau_sigma");