do not automatically create *_set_auxiliary_variables.m on preprocessor run
As *_set_auxiliary_variables.m is sometimes empty, it does not make sense to create it on every run. Only create it if something will be written to it.
This change requires either a flag in M_ to be tested every time the function is called in the code or to test exist('./*set_auxiliary_variables.m') == 2 and take the appropriate action in the code.