Computes the Matern kernel
k = matern(nu, sigma, d);
k = matern(nu, sigma, d) computes the value of the Matern kernel of
order nu and scales the output with factor sigma.
The inputs nu and sigma have to be scalars and the output has
the dimensionality of the third input argument. The implementation uses
the scaled version of besselk which has a more stable behaviour for
large orders of the Bessel function.
cov_matern, covgrad_matern, demogp_matern
Copyright (c) Lehel Csató (2001-2004)