Add the Substitution Fissile method : Allows to take a user defined fissile...

Add the Substitution Fissile method : Allows to take a user defined fissile composition from an infinite stock in case of the FabPlant can't manage to build a fuel (not enough fissile, ...)

git-svn-id: svn+ssh://svn.in2p3.fr/class@637 0e7d625b-0364-4367-a6be-d5be4a48d228