Add three overlapping generations macro models

[rickecon]

@dkgaraujo. This PR adds three large scale overlapping generations models to the table.

• OG-Core: This model is the core overlapping generations logic. We create country calibration repositories that use OG-Core as a dependency (e.g., OG-USA and OG-UK). However, OG-Core can be run on its own and calibrated to mimic any economy.
• OG-USA: Country calibration for United States, which uses OG-Core as a dependency. This project is fairly mature, and has been used in multiple peer-reviewed papers and policy papers.
• OG-UK: Country calibration for United Kingdom, which uses OG-Core as a dependency. This project is fairly new and is still under development.

[rickecon]

@dkgaraujo . I didn't see any action or comment on this PR for 18 days. So I am closing it.

[rickecon]

Thanks, @dkgaraujo.