[Tree] Delete virtual copy and copy assignment methods in TTree

The `TTree` class already deletes the copy constructor and the copy
assignment, but users might still be tempted to use the virtual copy
constructor or copy assignment provided by `TObject::Clone()` and
`TObject::Copy()`. But these implementations also make no sense to the
TTree.

To avoid user confusion, this commit suggests to override these methods
in TTree such that they thow an exception and tell you what to use
instead.

Prevents reports this one in the future:

  * https://its.cern.ch/jira/browse/ROOT-9111

Author: guitargeek

tree/tree/inc/TTree.h

@@ -340,6 +340,9 @@ class TTree : public TNamed, public TAttLine, public TAttFill, public TAttMarker
    TTree(const TTree& tt) = delete;
    TTree& operator=(const TTree& tt) = delete;
 
+   TObject *Clone(const char *newname="") const override;
+   void     Copy(TObject &object) const override;
+
    virtual Int_t           AddBranchToCache(const char *bname, bool subbranches = false);
    virtual Int_t           AddBranchToCache(TBranch *branch,   bool subbranches = false);
    virtual Int_t           DropBranchFromCache(const char *bname, bool subbranches = false);

tree/tree/src/TTree.cxx

@@ -466,6 +466,7 @@ End_Macro
 #include <climits>
 #include <algorithm>
 #include <set>
+#include <stdexcept>
 
 #ifdef R__USE_IMT
 #include "ROOT/TThreadExecutor.hxx"
@@ -1060,6 +1061,17 @@ TTree::~TTree()
    }
 }
 
+TObject *TTree::Clone(const char *) const
+{
+   throw std::runtime_error("TObject::Clone() is not supported by TTree! Use TTree::CloneTree().");
+}
+
+void TTree::Copy(TObject &) const
+{
+   throw std::runtime_error(
+      "TObject::Copy() is not supported by TTree! Use TTree::CopyTree() or TTree::CopyEntries().");
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Returns the transient buffer currently used by this TTree for reading/writing baskets.
 
@@ -3205,7 +3217,7 @@ TTree* TTree::CloneTree(Long64_t nentries /* = -1 */, Option_t* option /* = "" *
 
    // Note: For a chain, the returned clone will be
    //       a clone of the chain's first tree.
-   TTree* newtree = (TTree*) thistree->Clone();
+   TTree* newtree = (TTree*) thistree->TNamed::Clone();
    if (!newtree) {
       return nullptr;
    }