Missing access to YANG substatements in Python Path API

Currently, the Python Path API gives access to the keyword and argument of YANG statements ([`ydk.path.Statement`](http://ydk.cisco.com/py/docs/api/path/statement.html)) of a schema node. However, it gives no access to substatements. For example, there is no way to find out if a node is a configuration node or a mandatory node. In the case of list nodes, there is no way to find the keys.