ScheduleTree: make "write" method abstract

It does not make sense to output a tree node for which we don't know the
specific type.  This change also makes the ScheduleTree class abstract,
making it impossible to construct instances of this class even
privately.

Author: ftynse

tc/core/polyhedral/schedule_tree.h

@@ -463,9 +463,7 @@ struct ScheduleTree {
 
   // Write the textual representation of the node to "os".
   // Note that this function does _not_ output the child trees.
-  virtual std::ostream& write(std::ostream& os) const {
-    return os;
-  }
+  virtual std::ostream& write(std::ostream& os) const = 0;
 
   //
   // Data members