implementing TTree friends as an alternative to hard cloning

• ensure the same order of the input files (although only done for 3 digits)
  • check with HTCondor
• define a new explicit option, namely -F
• extend example02
• write a proper test to ensure that the physics result is identical
• benchmark on large scale examples
• make sure that friends' friends are also friends (otherwise import the list of friend)

To make it work, one key feature was to remember to call tIn->SetBranchStatus() and tOut->Branch() with the same arguments (note: this makes the code a bit heavier... one should think of a more concise way).