change the status of variables to a model
model2=changesym(model,arg1,...,argn)
* model = a model typed list created by the function create_model
* arg1,...,argn = a number of paired arguments with the following sequence:
'type of variable', vector_of_variables_names with 'type of variable' = 'endogenous', 'exogenous' or 'residuals' or 'notransf' if the user does not want to transform for estimation and simulation
* model2 = the new model, with the new endogenous, exogenous and residuals and coresponding fields adjusted