Consider GMM lower limits at short T

[pmpowers-usgs]

CB14 imposes a rule that ground motions at short periods (T<0.3s) are not allowed to be less than PGA. A recent email from another NGAW2 developer states that their model should impose the same rule, but this was never stated in their publication. Should we apply this rule (with developers blessing) across all USGS GMM implementations? or just make the change to NGAW2 models? or do nothing.