Add constructors from intializer lists and helpers

[JohanMabille]

Batch constructor taking an initializer list should be available. Helpers allowing to build batches from initializer lists without knowing their real type should be implemented too.

[serge-sans-paille]

For the record, something similar (using boost.simd syntax) works just fine for my use case

  template<class P, size_t... Is>
  P iota(std::index_sequence<Is...>) {
    return {static_cast<typename P::value_type>(Is)...};
  }

  template<class P>
  P iota() {
    return iota<P>(std::make_index_sequence<P::static_size>());
  }

called as

iota<boost::simd::pack<T>>()

[HadrienG2]

Since you know the length at compile time, shouldn't you take an std::array<T, N> instead of an initializer_list? Seems to me that it would avoid brace initialization confusion with the constructor taking N scalars...

[serge-sans-paille]

At some point we've been supporting initializer lists, but they turned out to be confusing for users. This got removed by 3ce1809f417703bc804de4fc1b3310c678b823d2 as an answer to #775.

Current supported constructor has the following signature:

template <class T, class A>
    template <class... Ts>
    inline batch<T, A>::batch(T val0, T val1, Ts... vals) noexcept

And it performs compile-time checking on the number of elements to make sure they match batch size.