[FEATURE] Implement SkillValue class

[wsbrenk]

and use it in rating prediction and other places instead of Double

class SkillValue(var value: Double) {
    var displayValue: Double
        get() {
            if (value > 0) return value + 1;
            return 0.0;
        }
        set(v) {
            value = max(0.0, v - 1)
        }

    fun increment(v: Double) {
        value += v
    }
}