A todo list for the state space models

1) Documentation for the functions

2) implementation for 
Matern32 - Done
Matern52
exponential - Done
bias
white
rbf
periodic Materns >???
periodic rbf

3) Addition and multiplication
addition - computing the sde variables is done, needs brushing
derivatives

multiplication
F = kron_sum(F1, F2)
L, Qc, H P_inf: kron_prod
derivatives!

4) Examples. Just a draw from a mater32. Show that the standard method gives the same result as the SS

5) Tests. Ensure that the log-likelihood is the same for the standard as the SS
          same for the derivatives.
          checkgradding
          check adding and multiplication works for various combinations

6) plotting
plot the kalman fitered result, and the smoothed result.
Copy AS's animation
posterior samples