Fix Key.NonTraditional model

[jsbean]

Currently, Key.NonTraditional is defined as such:

struct NonTraditional {
    public var step: Step
    public var alter: Double
    public var accidental: AccidentalValue
}

However, NonTraditional should hold an array of these values (step, alter, and accidental), instead of a single instance. Like so:

struct NonTraditional {
    struct AlteredTone {
        let step: Step
        let alter: Double
        let accidental: AccidentalValue
    }
    let alteredTones: [AlteredTone]
}

This should resolve the currently non-parsing 13c-KeySignatures-NonTraditional.xml test.