Change the mechanism to manage the created models.

This update has been requested by GN and this is his description:

Our current strategy  is that if a model file XXX specified  in the config file with same name exists, we first copy the old XXX to XXX_old.
If XXX_old already exists, we delete it, and then rename XXX to XXX_old, so that we always keep only one XXX_old.
The latter just to be on the save side and save at least the latest existing model file of same name as that used in the  config file.
Merge pull request #347 from rzanoli/master

Change the mechanism to manage the created models.