Return an |OpcPackage| instance loaded with the contents of
        *pkg_file*.

def open(cls, pkg_file):
        """
        Return an |OpcPackage| instance loaded with the contents of
        *pkg_file*.
        """
        pkg_reader = PackageReader.from_file(pkg_file)
        package = cls()
        Unmarshaller.unmarshal(pkg_reader, package, PartFactory)
        return package