gammaKernelTau evaluates the scale factor tau_n for the gamma kernel using the
exact expression derived from the covariance function.
gammaKernelTau(n, alpha, lambda)a positive integer indicating the number of observations per unit of time.
the smoothness parameter, alpha, for the gamma kernel.
the exponent parameter, lambda, for the gamma kernel.
The function returns the scale factor (tau_n) for the gamma kernel with
parameters alpha and lambda, observed at frequency n per unit of time.