Preprocessor update

– Fix interaction of temporary terms with steady_state operator.
  Incidentally, this implies that external functions can no longer be used inside
  steady_state operators.
– JSON output: remove duplicate “rhs” field when RHS of equation is non-zero
  constant
– Bytecode: remove hack for exogenous deterministic index

preprocessor @ 93ef4c6f

-Subproject commit ed22904568d1b69faf0d36ccbcfdac95d718da1a
+Subproject commit 93ef4c6febaff596d4593f06f1499a7315811074