Method advance

include/boost/histogram/axis/iterator.hpp:31–31

  void increment() noexcept { ++idx_; }
  void decrement() noexcept { --idx_; }
  void advance(int n) noexcept { idx_ += n; }
  int distance_to(const iterator_over& other) const noexcept {
    return other.idx_ - idx_;
  }