New CAV models

[emthompson-usgs]

Hi @mmpagani and @micheles,

@kslytherin and I are going to start working on a few new CAV models to contribute, specifically:

• Bullock et al. 2019
• Bullock et al. 2021
• Liu and Macedo, 2022

This issue's main purpose is to inform you about our plans and try to avoid duplicating effort, which happened for the Campbell and Bozorgnia model.

Also, the Liu and Macedo paper includes a conditional model, so I wanted to ask what the strategy for these models in OQ will be. If @g-weatherill's implementation in this PR has issues that prevent it from being merged, could we discuss how it needs to be modified to move things forward?

Thanks.