add PGA bias correction coeff into CoefficientsAkBias as temporary improvement, this will break tests

Update MA05 in PRVI subduction GMM groups

See merge request !408

add beta PRVI GMM groups

See merge request !407

KBCG_20 sigma update for Cascadia

See merge request !406

Nga east adjustment update

See merge request !405

Static method to parse gmm tree

See merge request !404

Beta implementation of Gomez 2022

Closes #232

See merge request !396

Max M for rate calc 9.4 --> 9.5

See merge request !403

Grid table focal mech update

Closes #237

See merge request !402

initial implementation of Motazedian-Atkinson 2005 GMM for PR. test results are unverified

See merge request !399

delegates interpolation of MPRS coefficients to a MA05 subclass, which uses the native coefficients for better interpolation, to compute the reference GM to which BA08 site scaling is applied. this works but needs to be cleaned up.