Preprocessor update

— Bugfix for “diff” operator in static model
— Fill the “M_.aux_vars(:).orig_expr” field for all auxiliary
  variables (Closes: #773)

preprocessor @ d441ff05

-Subproject commit 044258b0a217bc6e87c08d97b343ed56b7ba9d6e
+Subproject commit d441ff059a57d3020b055a31ab45ac72668f782b