powerKernelTau evaluates the scale factor tau_n for the power law kernel using
numerical integration for the covariance term, and exact evaluation for the variance term.
powerKernelTau(n, alpha, beta)a positive integer indicating the number of observations per unit of time.
the smoothness parameter, alpha, for the power law kernel.
the exponent parameter, beta, for the power law kernel.
The function returns the scale factor (tau_n) for the power law kernel with
parameters alpha and beta, observed at frequency n per unit of time.