message: Prevent spawn in group for statically typed actors
diff: @@ -406,6 +406,8 @@ public:
  template <class T, spawn_options Os, class Iter, class... Ts>
  infer_handle_from_class_t<T>
  spawn_class_in_groups(actor_config& cfg, Iter first, Iter last, Ts&&... xs) {
  + static_assert(std::is_same<infer_handle_from_class_t<T>, actor>::value,
  + "Only dynamically typed actors can be spawned in a group.");
  check_invariants<T>();
  auto irange = make_input_range(first, last);
  cfg.groups = &irange;
  @@ -419,6 +421,8 @@ public:
  infer_handle_from_fun_t<F>
  spawn_fun_in_groups(actor_config& cfg, Iter first, Iter second,
  F& fun, Ts&&... xs) {
  + static_assert(std::is_same<infer_handle_from_fun_t<F>, actor>::value,
  + "Only dynamically actors can be spawned in a group.");
  check_invariants<infer_impl_from_fun_t<F>>();
  auto irange = make_input_range(first, second);
  cfg.groups = &irange;
  