epilogue block fixes/enhancements

Playing a bit with the epilogue block, I would suggest the following fixes/enhancements:

1. epilogue should honor trends defined for variables, so to ensure that leads/lags of variables are equally treated in model and epilogue blocks;
2. it would be good to store the names of the epilogue variables in M_, for example M_.epilogue_names, so that we can think of allowing post-processing routines to automatically recognize that such a name means something.