Preprocessor update

– Make preprocessor recognize default order=2 of stoch_simul
— Fix faulty logic in checks for mh_tune_jscale
— Fix handling of multiple PAC models
— PAC/MCE: improve informative message about # of aux. vars/eqs by adding the
  model name
— Fixes to include_eqs/exclude_eqs
— PAC model: rewrite the algorithm for detecting the target, to make it more
  robust
— PAC/MCE model: fix timing of target in PAC expectation term
— Add new “matched_moments” block