Preprocessor update

–  ramsey_policy: lift restriction that order must be ⩽ 2
–  Extend incompatibility with deterministic exogenous to ramsey_model command
–  Incorrect stochastic transformation with endo lead ⩾ 2 or exo lead ⩾ 1 in external functions
–  Crash when writing the (static) set_auxiliary_variables file in the presence of external functions