[N x D] vector, N - number of random variables, D - dimensionality of random variable
[N x N] covariance matrix
[N x D] vector, N - number of random variables, D - dimensionality of random variable
[M x D] vector, N - number of random variables, D - dimensionality of random variable
[N x M] covariance matrix
Covariance function that measures similarity between two points in some input space.