Small enrichment fixes

This MR contains two small bug fixes, first prevent particles that do not have neighbours to not do any stellar evolution and therefore feedback, allow gas to cool if there is enrichment because this can happen physically.

This MR works for the isolated galaxy example lowres64, with debugging checks, the full EAGLE model and -e at run time.

src/feedback/EAGLE/feedback.c

@@ -717,6 +717,14 @@ void compute_stellar_evolution(const struct feedback_props* feedback_props,
 
   /* Properties collected in the stellar density loop. */
   const float ngb_gas_mass = sp->feedback_data.to_collect.ngb_mass;
+
+  /* Check if there are neighbours, otherwise exit */
+  if (ngb_gas_mass == 0.f) {
+    feedback_reset_feedback(sp, feedback_props);
+    return;
+  }
+
+  /* Update the enrichment weights */
   const float enrichment_weight_inv =
       sp->feedback_data.to_collect.enrichment_weight_inv;
 
@@ -727,8 +735,7 @@ void compute_stellar_evolution(const struct feedback_props* feedback_props,
   feedback_reset_feedback(sp, feedback_props);
 
   /* Update the weights used for distribution */
-  const float enrichment_weight =
-      (enrichment_weight_inv != 0.f) ? 1.f / enrichment_weight_inv : 0.f;
+  const float enrichment_weight = 1.f / enrichment_weight_inv;
   sp->feedback_data.to_distribute.enrichment_weight = enrichment_weight;
 
   /* Compute properties of the stochastic SNII feedback model. */