Generate matrix-wise functions for two matrices using matern kernel.
kernel_matern(l, p, sigma)(numeric) A numeric number indicating the hyperparameter (flexibility) of a specific kernel.
(integer) For polynomial, p is the power; for matern, v = p + 1 / 2; for rational, alpha = p.
(numeric) The covariance coefficient for neural network kernel.
(function) A function calculating the relevance of two matrices.
Matern Kernels $$k_{Matern}(r)=\frac{2^{1-\nu}}{\Gamma(\nu)}\Big(\frac{\sqrt{2\nu r}}{l}\Big)^\nu K_\nu \Big(\frac{\sqrt{2\nu r}}{l}\Big)$$
The MIT Press. Gaussian Processes for Machine Learning, 2006.