This is a base class designed to handle the specific features of covariances needed for outerbase. Polymorphism allows for the implied methods to be used across several similar classes.
no returns, this is a class which contains methods
covf$hyphyperparameters for this specific correlation function
covf$lowbnd,covf$uppbndupper and lower bounds for the inputs to the covariance function.
covf$cov(x1,x2)returns the covariance matrix between two vectors of
inputs x1 and x2
covf$covdiag(x1)returns the diagonal of the covariance matrix between
x1 and itself
covf$cov_gradhyp(x1,x2)returns a cube of the gradient the cov
with respect to the covariance hyperparameters
derived class: covf_mat25,
covf_mat25pow,
covf_mat25ang