[RF] Implement RooLinkedList::contains()

This mimics what is done in the newer C++ standards for multiple
standard library classes, and also tells cppyy how to implement
`__contains__` in the Python proxy.

Author: guitargeek

roofit/roofitcore/inc/RooLinkedList.h

@@ -95,6 +95,9 @@ class RooLinkedList : public TObject {
     return _first ? _first->_arg : nullptr ;
   }
 
+  // For checking if the list contains an element with a given name from Python.
+  bool contains(const char* name) const { return find(name); }
+
   void RecursiveRemove(TObject *obj) override;
 
   void Print(const char* opt) const override ;