Correct a fat finger (from a confident copy past) causing the MURE2CLASS Utils...

Correct a fat finger (from a confident copy past) causing the MURE2CLASS Utils writing only one nuclei when using a MURE output with one cell

git-svn-id: svn+ssh://svn.in2p3.fr/class@456 0e7d625b-0364-4367-a6be-d5be4a48d228