Exponential(d, range = 1, alpha = 1/range, phi = 1)
Matern (d , scale = 1, range = 1,alpha=1/range,
smoothness = 0.5, nu= smoothness, phi=scale)phi* exp( -d/range)
Matern:
phi*con*(d^nu) * besselK(d , nu )
Matern covariance function transcribed from Stein's book page 31 nu==smoothness, alpha == 1/range
GeoR parameters map to kappa==smoothness and phi == range check for negative distances
con is a constant that normalizes the expression to be 1.0 when phi=1.0
and d=0.