Add SetSpectrumType function. The aims is to change the isotopic branching...

Add SetSpectrumType function. The aims is to change the isotopic branching ratio (for isomer production by reaction) according the type of neutron spectrum

git-svn-id: svn+ssh://svn.in2p3.fr/class@489 0e7d625b-0364-4367-a6be-d5be4a48d228