correct initialization of simulation variables
- the current implementation process on the fly the initialization information provided by initval, endval and shocks, creating along the way oo_.endo_simul, oo_.exo_simul and oo_.exo_det_simul (information provided by histval is now correctly stored in M_ before processing)
- this implementation is prone to errors and forces users to respect a strict order of declaration. Simple checks of compliance with this order prevents users some legitimate usage such as macro loops
- the solution is to record the initialization information in fields of M_ and to create/initialized oo_.endo_simul, oo_.exo_simul and oo_.exo_det_simul just before they are used.
- set_shocks, make_y and make_ex should be rewritten and, for the last two, their name changed.