ENH: (WIP) MVAR connectivity and example

Excellent suggestion! The autocorrelations are computed for lags up to the model order, which should be small relative to the number of time points (unless there are many epochs).

Anyway, I am inclined to drop Yule-Walker MVAR fitting altogether. The reason why it was implemented was to support generators to preserve memory. But you only need to preserve memory if you have many signals, in which case you need regularization - and I don't know how to regularize the Yule-Walker equations.

However, the similar ACM calculations are performed in SCoT to test the resiudals for whiteness, which requires many lags and might benefit from going Fourier :)