Integrate GEDA and Gaussian Autoregressive models

[cweniger]

In order to make that happen, we need to consolidate the Gaussian_Autoregressive interface. What about dropping the _X, and only keeping the module-based version? We can later add functionality to default to linear modules for L and Phi. Having two different versions of essentially the same function is probably not the best idea.

@NoemiAM, @FloList , any thoughts about this?

[NoemiAM]

@cweniger I agree we should drop one of the two since they are basically the same. But the consolidated version should contain both sampling methods: standard Cholesky decomposition and GEDA.