GARCHModel gradient method's return may error

[endymecy]

GARCHModel's gradient method return Array(alphaGradient * .5, betaGradient * .5, omegaGradient * .5). I think the return should be Array(omegaGradient * .5, alphaGradient * .5, betaGradient * .5 )

[sryza]

Thanks for catching this, I think you're correct. Would you like to submit a pull request for it? If not, I can make the fix myself.