flame_wave: error out if atm_delta is too small

If atm_delta is significantly smaller than the finest-level resolution,
the peak temperature gets reset to T_star instead of T_hi, and the hot
and cool zones end up with the same temperature profiles. I'm not
entirely sure how it happens, and it's probably fixable, but we
shouldn't be doing any actual runs with such a low resolution anyways.
I only ran into this when I was using a single level for local testing.

Exec/science/flame_wave/initial_model.H

@@ -236,6 +236,9 @@ generate_initial_model(const int npts_model, const Real xmin, const Real xmax,
     for (int i = index_base+1; i < npts_model; i++) {
 
        if ((model::profile(model_num).r(i) > xmin + model_params.H_star + 3.0_rt * model_params.atm_delta) && !flipped) {
+           if (i == index_base + 1) {
+               amrex::Error("atm_delta is too small for the grid spacing (at least one transition zone is required)");
+           }
            isentropic = true;
            flipped = true;