Implementation of long period M9 adjustments

Placeholder issue for determining implementation/consideration of M9 informed basin amplification factors in the vicinity of Seattle.

@mmoschetti

added 2023 GMM labels

mentioned in issue #169 (closed)

changed milestone to %Finalize draft NSHM 2023

Resolved in !312 (merged) and recent updates to nshm-conus e.g. ghsc/nshmp/nshms/nshm-conus!56 (merged)

closed