Message getters that return the field value instead of quickfix.Field types

Created by: cbusbey

eg,

//SetAdvId sets AdvId, Tag 2
func (m Advertisement) SetAdvId(v string) {  // accepts Go's primitive type
    m.Set(field.NewAdvId(v))
}


//GetAdvId gets AdvId, Tag 2
func (m Advertisement) GetAdvId() (f field.AdvIdField, err quickfix.MessageRejectError) { // does not return Go's primitive type but Field
    err = m.Get(&f)
    return
} 

Getters and setters should be symmetric

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