Add USGS Alaska bias adjustment for NGASUB interface GMMs

[CB-quakemodel]

Add the Alaska bias adjustment capability for global variants of the interface NGASUB GMMs as used in the new USGS Alaska model. The development and rationale for this adjustment is described in the Powers et al. (2024) Alaska NSHM EQ Spectra paper.

Example of impact for the global variant of the Parker et al (2020) NGA sub GMM for some interface events:

To-do

• Add for Kuehn 2020 (there is also a non-linear site adjustment to sigma for this GMM - best put in a separate PR - it is NOT required/approved for use in the conterminous model, just Alaska)
• Add unit tests.

Also some moving of the lin2011 GMM (it should not have it's own folder in the gsim library if it has no extra files)