Add Shahjouei and Pezeshk 2016 CEUS GMM, which is an update to the NGA_EAST_SEED_SP15 model

Created by: ashumway-usgs

Shahjouei and Pezeshk have updated their NGA-East Seed Model (SP15) with a new model. Their BSSA paper provides a functional form and coefficient tables for median ground motions and sigma.