[DF] Add support for varied `Snapshot`

[eguiraud]

In presence of systematic variations registered with RDF with Vary, a Snapshot call could produce a TTree that contains extra branches for the different systematic variations.

To do

• [x] decide between extra branches and extra trees
• [ ] figure out how the interaction with VariationsFor works
• [ ] decide naming convention for varied branches

Then for the actual implementation:

• [ ] add template specialization for RVariedAction<SnapshotHelper> and RVariedAction<SnapshotHelperMT> that does the right thing
• [ ] fix RVariedAction::GetSampleCallback so that it does not only take the callback of the first helper (this might turn out to be an orthogonal issue, but it's in principle relevant as SnapshotHelper[MT] makes use of the sample callback mechanism