add ProgramTreeIdentity and make ProgramTree inherit from interface.RootEntity
add ProgramTreeIdentity and make ProgramTree inherit from interface.RootEntity

uclouvain/OSIS-Louvain,uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/osis,uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/osis