logdensity.gp: Calculates log prior density of a spectral GP object
Description
Calculates the log prior density of a spectral GP object as the log
prior density of the basis coefficients, based on the prior variances
and a prior of independent Gaussians.
The log density is calculated based on the real and imaginary
components of the basis function coefficients, but only those
coefficients that are not determined as the complex conjugates of
other coefficients. The density function is that the coefficients
are IID normal with mean zero and prior variance based on the
spectral density and correlation parameters.