Merge branch 'nga-sub-parker' into 'master'

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

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

src/main/resources/gmm/coeffs/PSHAB20_interface.csv

src/main/resources/gmm/coeffs/PSHAB20_slab.csv

src/test/java/gov/usgs/earthquake/nshmp/gmm/PSHAB20.java

src/test/resources/gmm/PSHAB20_inputs.csv

src/test/resources/gmm/PSHAB20_results.csv