Add autoregressive model to V4

[mjhajharia]

created this issue linked to the project for porting time series distributions to v4

[mjhajharia]

related to #4642

[ricardoV94]

@mjhajharia Can you give a more descriptive name to this issue?

[canyon289]

Add at minimum AR1 model to PyMC v4, ideally add AR(n) to facilitate addition of ARMA model