Preliminary NGA-Subduction implementation

Create preliminary implementations of NGA-Subduction GMMs. Two models are under consideration; both include epistemic uncertainty; both include slab and interface forms. Earthquake Spectra references for the models below are expected, but likely won't be available for some months.

• Parker et al. (PEER 2020/03) [JA]
• Kuehn et al. (PEER 2020/04) [JA]
• Abrahamson and Gulerce (PEER 2020/25) [PP]
• Also review NGA-Subduction (for USGS) and #53 (closed) [PP]