Avoid FP overflow during Detonation initialization

I was getting errors here when running with `amrex.fpe_trap_overflow=1`.

Exec/science/Detonation/problem_initialize_state_data.H

@@ -22,7 +22,14 @@ void problem_initialize_state_data (int i, int j, int k,
 
     state(i,j,k,URHO) = problem::dens;
 
-    Real sigma = 1.0_rt / (1.0_rt + std::exp(-(c_T - xcen)/ width));
+    Real sigma_arg = -(c_T - xcen) / width;
+    // need to avoid FP overflow for sigma_arg >= 709
+    // 1/(1 + exp(100)) ~= 3e-44, which is much smaller than machine epsilon
+    if (sigma_arg < 100) {
+        sigma = 1.0_rt / (1.0_rt + std::exp(sigma_arg));
+    } else {
+        sigma = 0.0_rt;
+    }
 
     state(i,j,k,UTEMP) = problem::T_l + (problem::T_r - problem::T_l) * (1.0_rt - sigma);