vector spatial autocorrelation

Experiment using the PySAL Moran's I class to template a vector autocorrelation measure from Liu, Tong, and Liu (2014).

Included a "slag" method because it needed to be customized to accomomdate array nature of "z". Perhaps there is a better solution that does not require a custom method.

Todo:

• Add routines for simulating random permutations from Liu, Ting, and Liu (2014) for hypothesis testing.