Convert leafs to model type in YDK-Py

Since YDK-Py 0.6.0, leafs have type [Yleaf](http://ydk.cisco.com/py/docs/api/types.html#ydk-types).  This type requires conversion (e.g. to int, to float, etc).  Using properties, a reference to a leaf could return data in the same type define in the data model.  That automatic conversion will improve usability.