Closes the current functional group by writing its footer segment.
public func createFunctionalGroupFooter() throws -> Void
The CreateFunctionalGroupFooter method will write any existing segments to the output stream and then generate and write the functional group closing segment (GE or UNE). If there is an open transaction set, CreateFunctionalGroupFooter will first call CreateTransactionFooter to close it. If there is no functional group open, an exception will be thrown.