fix(topological_sort): rm nonconforming concurrent

In both `do concurrent` blocks inside the topological_sort
function, each iteration conditionally references an "order"
array that is defined in other iterations.  Fortran disallows
such code for variables that have no specified locality and adding
locality would not help: the algorithm needs to accumulate values
across iterations.  A different algorithm would need to be chosen
to parallelize the topological sort.

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 concurrent(v = 1:size(dag%vertices))
+      do 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
@@ -60,7 +60,7 @@ pure recursive function depth_first_search(v, d, o) result(hybrid)
       associate(dependencies => dag%depends_on(v))
         block
           integer w
-          do concurrent(w = 1:size(dependencies))
+          do w = 1, size(dependencies)
             associate(w_dependencies => dependencies(w))
               if (.not. any(w_dependencies == hybrid%s)) hybrid = depth_first_search(w_dependencies, [d, v], hybrid%o)
             end associate