Upgrade and refactor the TThreadedObject class and tutorial

o Properly put in the ROOT directory the TThreadedObject header
o Remove the ThreadIndex header as not necessary anymore
o Refactor and extensively document the TThreadedObject class
  - Better handling of size of slot array
  - Correct handling of pointers
  - Add the Get and GetAtSlotUnchecked methods
  - Move the assigning of slot index from an external helper to the class itself
o Update the tutorial and its documentation