Added support for old field groups without grouped location rules

Signed-off-by: Elliot Condon <e@elliotcondon.com>
Fixed potential issue with old field group code and incorrect indexed keys

Signed-off-by: Elliot Condon <e@elliotcondon.com>
Added public acf_add_local_field function

Signed-off-by: Elliot Condon <e@elliotcondon.com>