Unified exact propagation of uncertainty for general covariance function

[martinstep]

Exact propagation has an analytical solution if the integral of the kernel times a Gaussian function can be computed analytically. There is not a solution for unifying the calculation of exact propagation (in a single script) for such class of covariance functions.

More plausible solutions exist:

1. Each script is written on its own
2. There is a single script for propagation, but contains separated numerical recipes for various covariance functions
3. There is a single script for propagation that requires specific properties of propagation to be available inside each covariance function, for example, the integral of the kernel times a Gaussian function is already available inside the covariance function script.