Updated AG20 coefficients from latest EQSpectra electronic supplement.

- coefficients a1, cAk, and cCasc have 15 decimal places in EQS xlsx file, rounded to 6 decimal places
- coeffs a28, a29, a31, a32, a33, a34, a35, a36, and a37 had one or more values truncated, so additional decimal places are now included
- coeff a16 is not used in nshmp-lib code (Taiwan-specific) and has been removed
- coeffs e0, e1, and e2 are not referenced in the code or the EQS paper and have been removed