added clone functions for nodes

Since nodes are are extremely derivative types, we use a clone/copy function
to produce new types of the derived class easily.

app/node/block/gap/gap.cpp

@@ -24,7 +24,7 @@ GapBlock::GapBlock()
 {
 }
 
-Block *GapBlock::copy()
+Node *GapBlock::copy()
 {
   GapBlock* c = new GapBlock();
 

app/node/block/gap/gap.h

@@ -32,7 +32,7 @@ class GapBlock : public Block
 public:
   GapBlock();
 
-  virtual Block * copy() override;
+  virtual Node * copy() override;
 
   virtual Type type() override;