760: [fixingDates] should have [relativeDates] not [relativeDate]
Currently the [fixingDates] under the interest leg is of type [AdjustableDatesOrRelativeDateOffset], which has a choice element with [adjustableDates] and [relativeDate]. The latter should be [relativeDates] of type [RelativeDates] rather than [relativeDate] of type [RelativeDateOffset], as they are expected to result in multiple dates. (Note that this change will not be backward-compatible.)