Make returning metadata an open option

In the new version 2.2.0-1, the development of metadata reshaping makes metadata correct but also makes the function slower. Since a certain startR version, to ensure metadata are correctly returned, Start() automatically adds items to return_vars or change the values of return_vars under several conditions, including if the dimension is: (1) assigned by values, not indices (2) reordered (3) dependent on other dims (4) reshaped

To improve efficiency, it is better to remove these obligations. If metadata is needed, users should try to write the correct Start call to obtain them; if users want to have a better performance, they can check the data beforehand and not get metadata along with data in the computation.