Support weekly direct debit

CiviCRM specifies a weekly membership/recur as "Every 7 day". But GoCardless doesn't understand that this is "Every 1 week". I had a similar problem in Smartdebit and ended created a (probably overcomplicated) function to map params: https://lab.civicrm.org/extensions/smartdebit/blob/master/CRM/Core/Payment/Smartdebit.php#L508-521