AG20 PGA rock calculated considering basin term but shouldn't be

Abrahamson and Gülerce (2020) incorrectly passes in z2.5 when calculating PGA rock; should be Double.NaN. This guidance was absent in the initial PEER report but was added to the final Spectra paper.

added Bug GMM labels

mentioned in merge request !423 (closed)

This change does not impact test results or hazard given other conditionals

mentioned in merge request !430 (merged)

mentioned in commit e64879c7

closed with merge request !430 (merged)