message: First try at creating operator[]. Not compiling.
diff: @@ -384,6 +384,30 @@ public:
  return ret;
  }
  + /*!
  + * \brief Finds reference to value of key-value pair mapped to supplied pair, and performs insertion if
  + * item does not already exist in Map instance. For reading, modification, and insertion into Map instance.
  + *
  + * \param [in] key the key used to index into the Map to find/store item
  + *
  + * \note Major deviation from STL unordered_map in that this function is unsafe, due to inability to internally
  + * call rehash function. User must be careful to ensure integrity of Map with repeated use of this function.
  + *
  + * \return A reference to the value of key-value pair in the Map instance that is associated with supplied Key.
  + *
  + */
  + T& operator[](Key key)
  + {
  + axom_map::pair<Key, T> ins_result = insert(Key, T{0});
  +
  + //Add a way to make this more efficient than an access like this -- maybe keep an m_end in the Map.
  + if(ins_result == m_buckets[0].m_end){
  + return NULL;
  + }
  +
  + return &(ins_result.first->value);
  + }
  +
  /*!
  * \brief Removes key-value pair with given key from the Map instance.
  *
  