Add more information about auxiliary variables to M_.aux_vars
For auxiliary variables, the Dynare preprocessor appends the new equations after the ones provided by the user. Currently, the easiest way to see what they contain is to use the `write_latex_{static,dynamic}_model' commands. This will also reveal the match between auxiliary variables and what they replace. But accessing this information is rather cumbersome.
I would suggest to add the string of the substituted variable (which can already be written to TeX-code) as a field in M_.aux_vars, which together with the type stored there should be sufficient for full identification.
This would also allow for better dealing with lead variables, because an auxiliary variable can correspond to a more complex expression like E_t(x_{t+2}*y_{t+2}), which can only be replaced by a single variable (because of the expectancy operator).