feat(geo): Add Gen3 hierarchy search to lowestTrackingVolume

This does not affect the existing Gen1 hierarchy search, and only kicks
in when the Gen1 content is exhausted or not present at all.

Core/src/Geometry/TrackingVolume.cpp

@@ -96,6 +96,13 @@ const TrackingVolume* TrackingVolume::lowestTrackingVolume(
     }
   }
 
+  // @TODO: Abstract this into an accelerateable structure
+  for (const auto& volume : volumes()) {
+    if (volume.inside(position, tol)) {
+      return volume.lowestTrackingVolume(gctx, position, tol);
+    }
+  }
+
   // there is no lower sub structure
   return this;
 }