Replace DAGNode class with OpNode, InNode, and OutNode classes

qiskit/dagcircuit/__init__.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2017.
+# (C) Copyright IBM 2017, 2021.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -25,6 +25,9 @@
 
    DAGCircuit
    DAGNode
+   DAGOpNode
+   DAGInNode
+   DAGOutNode
    DAGDepNode
    DAGDependency
 
@@ -37,7 +40,7 @@
    DAGCircuitError
 """
 from .dagcircuit import DAGCircuit
-from .dagnode import DAGNode
+from .dagnode import DAGNode, DAGOpNode, DAGInNode, DAGOutNode
 from .dagdepnode import DAGDepNode
 from .exceptions import DAGCircuitError
 from .dagdependency import DAGDependency