static func save() {
    print("starting save...")
    let DocumentsDirectory = FileManager().urls(for: .documentDirectory, in: .userDomainMask).first!
    let ArchiveURL = DocumentsDirectory.appendingPathComponent("volumesData")
    NSKeyedArchiver.archiveRootObject(VolumeTableViewController.volumesArray, toFile: ArchiveURL.path)
    print("Ending save...")
}

init (volumeNumber: Int, completed: Bool, questionsData: [QA]) {
    self.volumeNumber = volumeNumber
    self.completed = completed
    self.questionsData = questionsData
}

// MARK: NSCoding
public convenience required init?(coder aDecoder: NSCoder) {

    let volumeNumber = aDecoder.decodeObject(forKey: "volumeNumber") as! Int
    let completed = aDecoder.decodeObject(forKey: "completed") as! Bool
    let questionsData = aDecoder.decodeObject(forKey: "questionsData") as! [QA]

    self.init(volumeNumber: volumeNumber, completed: completed, questionsData: questionsData)
}

func encode(with aCoder: NSCoder) {
    aCoder.encode(volumeNumber, forKey: "volumeNumber")
    aCoder.encode(completed, forKey: "completed")
    aCoder.encode(questionsData, forKey: "questionsData")
}

// Create standard initiator
init(questionsText:String, answerText: [String], correctAnswer: [Bool], selectedAnswer: [Bool?]) {
    self.questionsText = questionsText
    self.answerText = answerText
    self.correctAnswer = correctAnswer
    self.selectedAnswer = selectedAnswer
}