Missing access to SchemaNode children in Path API

Currently, the Python Path API gives access to the root and parent of a [`ydk.path.SchemaNode`](http://ydk.cisco.com/py/docs/api/path/schema_node.html). However, there is no method to get a schema node's children like there is for a data node (using the `ydk.path.DataNode.get_children()` method).


