SteamGenerator : delete _4SG ? And remplace the _1SG with the one with purge (work done in Chimiscope) : model + SimpleExample

SteamGenerator_1SG should be enough for all type of SteamGenerator. Maybe we should delete the SteamGenerator_4SG (1 additional SingularPressureLoss). Also, SteamGenerator_1SG has been modified to include purge (APG) : Model and SimpleExample are working in Chimiscope project.