[Tutorials] Use in RDF/Multicore tutorials the new TFile::Get<T> method

in order to make of this code the most up-to-date set of examples possible.

tutorials/dataframe/df007_snapshot.C

@@ -60,8 +60,7 @@ int df007_snapshot()
 
    // Open the new file and list the columns of the tree
    TFile f1(outFileName);
-   TTree *t;
-   f1.GetObject(treeName, t);
+   auto t = f1.Get<TTree>(treeName);
    std::cout << "These are the columns b1, b1_square and b2_vector:" << std::endl;
    for (auto branch : *t->GetListOfBranches()) {
       std::cout << "Branch: " << branch->GetName() << std::endl;
@@ -75,7 +74,7 @@ int df007_snapshot()
 
    // Open the new file and list the columns of the tree
    TFile f2(outFileNameAllColumns);
-   f2.GetObject(treeName, t);
+   t = f2.Get<TTree>(treeName);
    std::cout << "These are all the columns available to this tdf:" << std::endl;
    for (auto branch : *t->GetListOfBranches()) {
       std::cout << "Branch: " << branch->GetName() << std::endl;

tutorials/multicore/imt001_parBranchProcessing.C

@@ -23,8 +23,7 @@ int imt001_parBranchProcessing()
    auto file = TFile::Open("http://root.cern.ch/files/h1/dstarmb.root");
 
    // Get the tree
-   TTree *tree = nullptr;
-   file->GetObject<TTree>("h42", tree);
+   auto tree = file->Get<TTree>("h42");
 
    const auto nEntries = tree->GetEntries();
 

tutorials/multicore/mp103_processSelector.C

@@ -42,8 +42,7 @@ int mp103_processSelector()
 // #define __reproduce_davix
 #if defined(__reproduce_davix)
    auto fp = std::make_unique<TTree>(TFile::Open(file0));
-   TTree *tree;
-   fp->GetObject("h42", tree);
+   auto tree = fp->Get<TTree>("h42");
 #endif
 
    ROOT::TTreeProcessorMP pool(3);

tutorials/multicore/mt102_readNtuplesFillHistosAndFit.C

@@ -46,8 +46,7 @@ Int_t mt102_readNtuplesFillHistosAndFit()
    // We define our work item
    auto workItem = [&histograms](UInt_t workerID) {
       TFile f(Form("mt101_multiCore_%u.root", workerID));
-      TNtuple *ntuple = nullptr;
-      f.GetObject("multiCore", ntuple);
+      auto ntuple = f.Get<TNtuple>("multiCore");
       auto &histo = histograms.at(workerID);
       for (auto index : ROOT::TSeqL(ntuple->GetEntriesFast())) {
          ntuple->GetEntry(index);