Method add

include/boost/histogram/storage/array_storage.hpp:79–82

  template <typename U>
  void add(std::size_t i, const U& x) noexcept {
    BOOST_ASSERT(i < size());
    array_[i] += x;
  }

  const_reference operator[](std::size_t i) const noexcept {
    BOOST_ASSERT(i < size());