Add the arma11 model

[Ch0ronomato]

Adds the arma11 model to port to pymc!

[JTorgander]

Thank you for contributing! I have pushed the following modifications since the proposed version did not pass our consistency tests:

1. Adding a positive (HalfCauchy) constraint on the sigma parameter
2. Modifying the likelihood to use all observations instead of just the latest.

Also, you don't need to include the data JSON file since we already have the data stored in zip files.

[MansMeg]

@jakob will fix so only pymc3 model is included since we already have the data and draws.