message: BUGFIX: fix const access operator
diff: @@ -170,7 +170,7 @@ public:
  * \param [in] component the component to return.
  * \return a reference to the given component of the specified tuple.
  */
  - inline T & operator()( IndexType pos, IndexType component=0 ) const
  + inline const T & operator()( IndexType pos, IndexType component=0 ) const
  { return m_data[ pos * m_num_components + component ]; }
  /*!
  