Adjustments for US NSHM GMPEs

[emthompson-usgs]

For the 2023 hazard map, the USGS created an AK-specific bias adjustment for the AG20 model (implementation in Java is here). We would like to use this adjustment for ShakeMap and so we would like to contribute this adjustment to OQ. Before we get to work on it, we'd like to check to see if you have any guidance on how this should be implemented. It looks like it is just a period-specific additive factor. Thanks.