Nga-subduction Parker et al. 2020 - initial

Initial implementation of Parker et al. (2020) NGA-Subduction GMM

Regions: global, Alaska, Aleutians, Cascadia

Status:

• Global interface and slab validated against 'official' python code
• median and all uncertainty calcs (total aleatory, ϕS2S, ϕSS, epistemic)
• Official python code run with 'updated' interface coefficients
• Official python code has a slightly different equation for the Path term (this and original have been implemented)

Todo:

• adjust calculations for other regions (and Cascadia basins)
• expand test cases for all regions and source types

See: #78 (closed)