message: Operator[] that compiles. Has major, but expected shortcomings.
diff: @@ -398,12 +398,9 @@ public:
  */
  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;
  - }
  + //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});
  return &(ins_result.first->value);
  }
  