Nga-subduction Parker et al. 2020 - initial

See merge request !142

added uncertainty calcs and fixed median - validated at input for global interface and slab against official python code

fixed issues with median gm calc, used expected results from official python implementation (modified to use updated coeffs). uncertainty not yet implemented

initial implementation of PSHAB20. calc mean needs to be fixed, uncertainty not yet implemented. tests do not pass.

add coefficient tables. updated interface coefs from PMP. includes all regions, omits a0_Cascadia (NA)

Lib work

See merge request !141

Lib work

See merge request !140

Lib work

See merge request !139