refac(topological_sort): nested concurrency

Author: Damian Rouson

src/dag_s.f90

@@ -39,7 +39,7 @@ pure module function topological_sort(dag) result(order)
 
       searched_and_ordered = searched_and_ordered_t(s = [integer::], o = [integer::])
 
-      do v = 1, size(dag%vertices)
+      do concurrent(v = 1:size(dag%vertices))
         if (.not. any(v == searched_and_ordered%s)) &
           searched_and_ordered = depth_first_search(v, [integer::], searched_and_ordered%o)
       end do