Fix dynamic enum attributes created without labels #56

As stated in issue #56 (closed), when creating a dynamic attribute of type DevEnum, the attribute is created without any label. The problem lies in the function PyAttr::set_user_prop() called by the constructor of PyScaAttr, when a user_prop argument is passed. The properties are parsed and reassembled into a UserAttrProb object by the set_user_prop() function, that lacks support for the "enum_labels" property. That's why the attribute is created without labels.