Merge branch 'gmm-work' into 'main'

...in/java/gov/usgs/earthquake/nshmp/gmm/BooreEtAl_2014.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

...main/java/gov/usgs/earthquake/nshmp/gmm/NgaEast_2018.java

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

.../java/gov/usgs/earthquake/nshmp/gmm/PezeshkEtAl_2018.java

.../gov/usgs/earthquake/nshmp/gmm/ShahjoueiPezeshk_2016.java

src/main/resources/gmm/coeffs/PSBAH20_interface.csv

src/main/resources/gmm/coeffs/PSBAH20_slab.csv

src/main/resources/gmm/coeffs/PZCT18_M1SS.csv

src/main/resources/gmm/coeffs/PZCT18_M2ES.csv

src/main/resources/gmm/coeffs/PZCT18_sigmas.csv

src/test/java/gov/usgs/earthquake/nshmp/gmm/NgaEast.java

src/test/java/gov/usgs/earthquake/nshmp/gmm/NgaSubAG20.java

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

src/test/java/gov/usgs/earthquake/nshmp/gmm/NgaSubSlab.java

.../gov/usgs/earthquake/nshmp/gmm/PezeshkEtAl_2018Tests.java

src/test/resources/gmm/NGA_SUB_interface_results.csv

src/test/resources/gmm/NGA_SUB_slab_results.csv