TDF: improper handling of branches with leaflists

TDataFrame deduces he type of TTree branches at runtime using the following logic:

• if branch->InheritsFrom(tbranchelRef), then the type of the branch is branch->GetClassName()
• else we check the last character of branch->GetTitle(), expecting title to be of the form "title/X" where X is a character that designates the branch type
• else the type could not be deduced

There are some cases that are not covered by this logic:

1. if the title of the branch is set explicitly by users via branch->SetTitle(), the last character will not represent the type, or worse it could sometimes correspond to a completely different type, leading to errors
2. leaflists are not supported by TTreeReaderValue. I am not sure how well we deal with failures to read branches with leaflists (even leaflists of length one, where the variable name is different than the branch name)

Reproducer:

#include <ROOT/TDataFrame.hxx>

#include <TTree.h>

#include <TFile.h>

int main() {

  // create file

  {

    TFile file("f.root", "RECREATE");

    TTree t("t", "t");

    float f;

    auto b = t.Branch("f", &f);

    b->SetTitle("customtitle");

    t.Branch("dontknow", &f, "ff/F");

    t.Fill();

    t.Write();

  }

  // read and write file with TDF 

  ROOT::Experimental::TDataFrame d("t", "f.root");

  d.Snapshot("t", "out.root");

  return 0;

}