Preprocessor update

– Block decomposition: fix crash when a variable appears in an equation but has
  zero symbolic derivative
– Correctly clean up options list after a “pac_model” statement

preprocessor @ ed450726

-Subproject commit 576c50c465c7b9eadb8ff1e3ab4429103c8d1064
+Subproject commit ed450726969225397e27b283587ff2ae41dd3016