Incorrect stochastic transformation with endo lead ⩾ 2 or exo lead ⩾ 1 in external functions

If an endogenous with a lead ⩾ 2 or an exogenous with a lead ⩾ 1 appeared in
the argument(s) of a call to an external function, the auxiliary variable
transformation was incorrect (the variable was replaced inside the function
call, while it is the whole function call that has to be replaced).

This could lead to incorrect results in stochastic contexts, when the external
function is nonlinear.

(manually cherry picked from commit 712b11a0)