Improvement of friend tree functionnalities

ROOT
3

This can be written as:

// The next line will:
//    a. The load the proper file if mainTree or sideTree are TChain
//    b.  Set the cursor ( `ReadEntry` ) for both the main tree and the side tree 
//         (using the information from the TTreeIndex).
//    c. does NOT bring into memory the data from any of branches ( `TBranch::GetEntry` needs to be called next).
Long64_t localentry = mainTree->LoadTree(5); 
// Then you can load the data with:
mainTreeBranch->GetEntry(localentry);
sideTreeBranch->GetEntry( sideTreeBranch->GetTree()->GetReadEntry() ) ;