Fix growth neutrality correction in PAC equations.

- Force long run levels of the exogenous variables to be zero or g (the BGP growth rate of the LHS endogenous variable).
- Fix the correction of the correction for the share λ (aak γ).
- Provisions for the case where the long run level of the exogenous variable is
different from 0 or g (see tmp1 and ll which should be added to the growth
neutrality correction as a constant).

(cherry picked from commit a4423d734e1df1d4ee09c7225d7fd610e0d94cd1)