removed all dependence on NodeOutputs

If Nodes only have the one output, we don't need to do so much differentiation
between them. Previous iteration used outputs as like a distinct function
within a Node (e.g. length output would return one result, buffer output would
produce a different result - each run different code to produce their results).
Now in this iteration, it's more accurate to say a Node is just one function
(which seems more appropriate for a node system anyway).

app/node/block/block.h

@@ -63,9 +63,6 @@ public:
   void set_previous(Block* previous);
   void set_next(Block* next);
 
-  NodeOutput* buffer_output() const;
-  NodeOutput* block_output() const;
-
   const rational& media_in() const;
   void set_media_in(const rational& media_in);
 
@@ -104,9 +101,6 @@ protected:
   Block* next_;
 
 private:
-  NodeOutput* block_output_;
-  NodeOutput* buffer_output_;
-
   rational length_;
   rational media_in_;