Correct negative value when using « IV-=«  due to double precision. Correct a...

Correct negative value when using « IV-=«  due to double precision. Correct a bug due to internal time of some factories not defined when created. This was causing the code to make an initial stock to decay while we don’t want it to do so  

git-svn-id: svn+ssh://svn.in2p3.fr/class@412 0e7d625b-0364-4367-a6be-d5be4a48d228