Merge branch 'cleanup-ak-nga-sub' into 'main'

...gov/usgs/earthquake/nshmp/gmm/AbrahamsonGulerce_2020.java

src/main/java/gov/usgs/earthquake/nshmp/gmm/CombinedGmm.java

src/main/java/gov/usgs/earthquake/nshmp/gmm/Gmm.java

...in/java/gov/usgs/earthquake/nshmp/gmm/KuehnEtAl_2020.java

...n/java/gov/usgs/earthquake/nshmp/gmm/ParkerEtAl_2020.java

...n/resources/gmm/coeffs/nga-sub-ak-interface-bias-corr.csv

...t/java/gov/usgs/earthquake/nshmp/gmm/NgaSubInterface.java

src/test/resources/gmm/nga-sub-interface-inputs.csv

src/test/resources/gmm/nga-sub-interface-results.csv