Define set_state as pure

Co-authored-by: Jeremie Vandenplas <[email protected]>

Author: castelao

src/nf/nf_layer_submodule.f90

@@ -447,7 +447,7 @@ module subroutine set_params(self, params)
 
   end subroutine set_params
 
-  module subroutine set_state(self, state)
+  pure module subroutine set_state(self, state)
     class(layer), intent(inout) :: self
     real, intent(in), optional :: state(:)