Adding a new class : The CLASS BackEnd.

The aim of this class is to describe all the facility used in the backend of the fuel cycle.

the reactor output fuel facility is now a BackEnd, so it is possible to go directly to a stock, or to chain the Pool, if wanted.

CLSSObject and CLSSFacility also became CLASSObject and CLASSFacility respectively without any major change.

git-svn-id: svn+ssh://svn.in2p3.fr/class@250 0e7d625b-0364-4367-a6be-d5be4a48d228