[model] add recursive traversal of model nodes into `objectmodel.Node`

[apalala]

     def _children_of_type(self, atype: Type = Node, recurse: bool = False, seen: set|None = None) -> Iterable[Node]:
        if recurse and seen is None:
            seen = set()

        for child in self._children():
            if isinstance(child, atype):
                yield child
            if recurse and isinstance(child, Node) and id(child) not in seen:
                seen.add(id(child))
                yield from child._children_of_type(atype=atype, recurse=True, seen=seen)

[apalala]

def _children_of_type(self, atype: Type = Node, recurse: bool = False, seen: set|None = None) -> Iterable[Node]:
    if seen is None:
        seen = {id(self)}

    for child in self._children():
        if id(child) in seen:
            continue
        else:
            seen.add(id(child))

        if isinstance(child, atype):
            yield child

        if recurse and isinstance(child, Node):
            yield from child._children_of_type(atype=atype, recurse=True, seen=seen)

[apalala]

walkers.DepthFirstWalker is enough to handle this requirement