Gets or sets the creditor payment reference.
The reference is mandatory for SwissQR IBANs, i.e. IBANs in the range CHxx30000xxxxxx
through CHxx31999xxxxx.
If specified, the reference must be either a valid SwissQR reference (corresponding
to ISR reference form) or a valid creditor reference according to ISO 11649 (“RFxxxx”).
Both may contain spaces for formatting.