Adapt `ProcessSequence` so that if cx_select and cx_sum are zero, we don't...

Adapt `ProcessSequence` so that if cx_select and cx_sum are zero, we don't select any interaction instead of the first one.

Adapt `Cascade`. In case a discrete interaction has been selected, it will only delete the initial particle from the stack
if `interaction` or `decay` have actually returned that they have called an interaction process or a decay process.
If this is not the case, which means we did not call `doInteract` or `doDecay`, we will keep the particle on the stack.

corsika/detail/framework/core/Cascade.inl

@@ -299,15 +299,20 @@ namespace corsika {
 
     FourMomentum const projectileP4Post{particle.getEnergy(), particle.getMomentum()};
 
+    bool eraseParticle =
+        false; // only erase original particle if it decayed or interacted
+
     if (distance_interact < distance_decay) {
       CrossSectionType const total_cx_post = composition.getWeightedSum(xs_function);
-      interaction(secondaries, projectileP4Post, composition, total_cx_post);
+      eraseParticle = isInteracted(
+          interaction(secondaries, projectileP4Post, composition, total_cx_post));
     } else {
       [[maybe_unused]] auto projectile = secondaries.getProjectile();
 
       InverseTimeType const total_inv_lifetime_post =
           sequence_.getInverseLifetime(particle);
       if (decay(secondaries, total_inv_lifetime_post) == ProcessReturn::Decayed) {
+        eraseParticle = true;
         if (secondaries.getSize() == 1 &&
             projectile.getPID() == secondaries.getNextParticle().getPID()) {
           throw std::runtime_error(fmt::format("Particle {} decays into itself!",
@@ -317,7 +322,7 @@ namespace corsika {
     }
 
     sequence_.doSecondaries(secondaries);
-    particle.erase();
+    if (eraseParticle) particle.erase();
   }
 
   template <typename TTracking, typename TProcessList, typename TOutput, typename TStack>

corsika/detail/framework/process/ProcessSequence.inl

@@ -508,7 +508,7 @@ namespace corsika {
         }
 
         // check if we should execute THIS process and then EXIT
-        if (cx_select <= cx_sum) {
+        if (cx_select < cx_sum) {
 
           if constexpr (has_signature_cx1) {
             // now also sample targetId from weighted cross sections
@@ -594,7 +594,7 @@ namespace corsika {
         }
 
         // check if we should execute THIS process and then EXIT
-        if (cx_select <= cx_sum) {
+        if (cx_select < cx_sum) {
 
           if constexpr (has_signature_cx1) {