Deterministic models: replace exogenous with lead/lags by auxiliary variables

This brings those models in line with stochastic models.

However note that transformation is still not exactly the same on stochastic
and deterministic models, because there is no need to take into account the
Jensen inequality on the latter. In deterministic models, there is a one-to-one
mapping between exogenous with lead/lags and auxiliaries, while in stochastic
models, an auxiliary endo may correspond to a more complex nonlinear expression.