Function for expanding the parameters of the fixedvariance kernel.
As the parameters of the fixedvariance kernel are fixed, and thus
are not optimized, their values remain the same. This function is
written to comply with the gptk package.
fixedvarianceKernExpandParam(kern, params)GP kernel structure which contains the fixed variances.
Parameter values with which the kernel will be updated.
Return kernel with the updated parameter values.