Computing the Spherical covariance function for a scalar distance.
single_cs(d, sigsq, phi)a scalar representing the (gaussian) covariance between two
observations d apart of each other.
a scalar representing the distance on which it is desired to evaluate the covariance function.
the \(\sigma^2\) parameter from the Spherical covariance. function.
the \(\phi\) parameter from the Spherical covariance function, controls the range of the spatial dependence.
single_exp, single_matern,
single_matern3, single_matern5,
mat_cov