Add abstract InitNode

InitNode is an abstract class which is meant to be inherited
by all the initialization nodes (such as CameraInit), included
those that might be created by the user.

InitNode contains methods that can be reimplemented by the
children classes if necessary.

This abstract class allows to keep on using scripts such as
meshroom_batch without having to modify them specifically or
being limited to using a CameraInit node.

meshroom/core/graph.py

@@ -547,6 +547,14 @@ class Graph(BaseObject):
         nodes = [n for n in self._nodes.values() if n.nodeType == nodeType]
         return self.sortNodesByIndex(nodes) if sortedByIndex else nodes
 
+    def findInitNodes(self):
+        """
+        Returns:
+            list[Node]: the list of Init nodes (nodes inheriting from InitNode)
+        """
+        nodes = [n for n in self._nodes.values() if isinstance(n.nodeDesc, meshroom.core.desc.InitNode)]
+        return nodes
+
     def findNodeCandidates(self, nodeNameExpr):
         pattern = re.compile(nodeNameExpr)
         return [v for k, v in self._nodes.objects.items() if pattern.match(k)]