Make lerp, mean, geomean and stdev extensions of Math

[shallawa]

    result[Strings.json.score] = Statistics.geometricMean(testsScores);

can be

    result[Strings.json.score] = Math.geomean(testsScores);

And

    var colorStopStep = Utilities.lerp(gradientStep, -.1, .1);

can be

    var colorStopStep = Math.lerp(gradientStep, -.1, .1);