[TDF] Add to the TDataSource the FinaliseSlot method

this may be useful to allow users to perform operations when
work stealing happens in order to restore the state the "stolen" work item
found before starting.

tree/treeplayer/inc/ROOT/TDataSource.hxx

@@ -75,11 +75,15 @@ public:
    /// For some implementation it's necessary to know the number of slots in advance for optimisation purposes.
    virtual void SetNSlots(unsigned int nSlots) = 0;
 
-   /// \brief Convenience method called at the start of the data processing.
+   /// \brief Convenience method called at the start of the data processing associated to a slot.
    /// \param[in] slot The data processing slot wihch needs to be initialised
    /// \param[in] firstEntry The first entry of the range that the task will process.
    virtual void InitSlot(unsigned int /*slot*/, ULong64_t /*firstEntry*/) {}
 
+   /// \brief Convenience method called at the end of the data processing associated to a slot.
+   /// \param[in] slot The data processing slot wihch needs to be finalised
+   virtual void FinaliseSlot(unsigned int /*slot*/) {}
+
 protected:
    /// type-erased vector of pointers to pointers to column values - one per slot
    virtual std::vector<void *>