Support for adjacency matrices in autoregressive models

[adrianjav]

This is an implementation of #53, in which I add support to provide an adjacency matrix to the autoregressive models, instead of using the ordering between variables.

I have added some asserts to make sure I cover all corner cases (e.g., providing both an ordering and an adjacency matrix), and added tests to make sure that the zero-entries of the adjacency matrix are indeed zero when computing the Jacobian matrix.

I might have missed something, and I am happy to fix it if that's the case.

PS: I read the policy about commit messages a bit too late, we can merge all commits into a single one and write an appropiate commit message.