[ntuple, doc] Add 'cluster groups' to the V1 specification
[ntuple, doc] Add 'cluster groups' to the V1 specification

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root