Mark copy constructor and assignment of ModelTree as protected

This limits the risk of slicing.

The same is not possible for DataTree, since that base class is directly used
in some places.

src/ModelTree.hh

@@ -409,9 +409,11 @@ public:
             ExternalFunctionsTable &external_functions_table_arg,
             bool is_dynamic_arg = false);
 
+protected:
   ModelTree(const ModelTree &m);
   ModelTree &operator=(const ModelTree &m);
 
+public:
   //! Absolute value under which a number is considered to be zero
   double cutoff{1e-15};
   //! Compute the minimum feedback set