src/gray_core.f90: refine realtime termination check

In some very rare cases the residual power check can stop the
integration too early.

For example, this extremely unlikely chain of events was observed in a
few cases while performing a parameter sweep (~3000 simulations):

  1. n⊥ converges on wrong branch ⇒ α jumps ⇒ dP/ds jumps (common)
  2. dP/ds jumps close to the peak (unlikely)
  3. the peak of dP/ds is close to 0.6P₀ (unlikely)
  4. dP/ds jumps to dP/ds_max + 10⁻¹² ⇒ wrong peak found (very unlikely)
  5. peak is at exactly the current point
     ⇒ three-point interpolation fails (unlikely)

src/gray_core.f90

@@ -529,7 +529,16 @@ contains
             end do
           end if
 
-          if (params%raytracing%realtime .and. pow <= 0.4_wp_*p0ray(1)) then
+          if (params%raytracing%realtime) then
+            ! Check whether we are past the absorption peak
+            block
+              logical :: past_peak
+              integer :: tail
+              ! We assume so if dP/ds has been decreasing in the last 8 steps
+              tail = max(2, i-8)
+              past_peak = i > 8 .and. all(dpds(tail:i) - dpds(tail-1:i-1) < 0)
+
+              if (past_peak .or. pow <= 0.1_wp_*p0ray(1)) then
                 ! Compute the approximate position of the absorption peak
                 block
                   use utils, only : vmax, parabola_vertex
@@ -549,6 +558,9 @@ contains
                 exit
               end if
 
+            end block
+          end if
+
           ! print ray positions for j=nrayr in local reference system
           if(mod(i,params%output%istpr) == 0) then
             if(params%raytracing%nray > 1 .and. all(.not.iwait)) &