made center element actually be returned first (#1088)

## Technical Description

This PR fixes a bug in the unstructured interface where the center element (if included) could reside in any position in the returned neighbor list. However, we rely on the fact that this is always returned first (also in ICON).

### Testing

Tested in `icondusk-e2e`

### Dependencies

This PR is independent.

dawn/src/interface/atlas_interface.hpp

@@ -347,21 +347,26 @@ inline std::vector<int> getNeighbors(atlasTag, atlas::Mesh const& mesh,
   std::list<int> result;
 
   // we want to exclude the original element from the neighborhood obtained if we don't include the
-  // center. we can compare by the id, but only if targetType = startOfChain, since ids may be
-  // duplicated amongst different element types; e.g. there may be a vertex and an edge with the
+  // center for now. we can compare by the id, but only if targetType = startOfChain, since ids may
+  // be duplicated amongst different element types; e.g. there may be a vertex and an edge with the
   // same id.
   NotDuplicateOrOrigin<int> pred;
-  if(chain.front() == chain.back() && !includeCenter) {
+  if(chain.front() == chain.back()) {
     pred = NotDuplicateOrOrigin<int>(idx);
-  } else {
-    pred = NotDuplicateOrOrigin<int>();
   }
 
   // start recursion
   getNeighborsImpl(nbhTables, chain, targetType, {idx}, result);
 
   std::vector<int> resultUnique;
   std::copy_if(result.begin(), result.end(), std::back_inserter(resultUnique), std::ref(pred));
+
+  // center element (if included) needs to go first. so we need to re-insert it (not possible to
+  // achieve this via predicate since it uses a set internally)
+  if(includeCenter) {
+    resultUnique.insert(resultUnique.begin(), idx);
+  }
+
   return resultUnique;
 }