message: Implementing empty() and downgrading operator[] to read-only.
diff: @@ -428,11 +428,11 @@ public:
  * \return A reference to the value of key-value pair in the Map instance that is associated with supplied Key.
  *
  */
  - T& operator[](Key key)
  + const T& operator[](Key key)
  {
  //Since we can't throw an exception, and this needs to return a reference to a variable of type T,
  //the case where there's no room in the Map is woefully unhandled.
  - axom_map::Pair<Key, T> ins_result = insert(key, T{0});
  + axom_map::Node<Key, T> &ins_result = find(key);
  return &(ins_result.first->value);
  }
  @@ -504,6 +504,11 @@ public:
  * \return capacity the overall capacity of the Map instance.
  */
  int max_size() { return m_bucket_len * m_bucket_count; }
  + /*!
  + * \brief Checks if the container has no elements.
  + * \return true if the container is empty, false otherwise
  + */
  + bool empty() const { return (m_size == 0); }
  ///@}
  private:
  /// \name Private Map Methods
  