Fix memory backing for density_source

Sometimes get surprising results because a value in the state
is modified while sharing data with a local field.

src/evolve_density.cxx

@@ -203,6 +203,7 @@ void EvolveDensity::transform(Options& state) {
   } else {
     final_source = source;
   }
+  final_source.allocate(); // Ensure unique memory storage.
   add(species["density_source"], final_source);
 }