create a model associated to an equation of a model
model_eq=create_model_eq(model,name_eq,name_endo)
* model = a model typed list created by the function create_model
* name_eq = the name of the equation
* name_endo = the name of the variable in the equation that will be considered as endogenous
* model_eq = a model tlist, the model created from the selected equation