Preprocessor update

– Fix orig_lead_lag field for unary op auxiliary variables (Closes: #1837)
– In case of nested unary ops (e.g. log(log())), ensure that the definition of
  the external aux var references the internal aux var
– Fix orig_lead_lag field for diff lag aux vars
– Refactor the code that substitutes unary ops in model equations
– Refactor handling of var_expectation_model statements
– Add missing “time_shift” option to JSON output of var_expectation_model
– Add check to ensure that column indices in derivative matrices do not overflow

preprocessor

@@ -1 +1 @@
-Subproject commit d149d3a76c7527608e55b7b73b6509dc3b4af097
+Subproject commit 5e8b478ccf795b4b4684b3c89e305cc083e3f0a3