Have GMM services return PGV if available

Currently services for GMvR and GMvM can return PGV values.
In Response Spectrum app, PGA is encoded as 0.001 SA, which is awkward.

• Update spectra service data object to be "PGA": value, "PGV": null, "SA": { "xvalues": [ ...], ...
• Update nshmp-apps for new response structure (see nshmp-apps#169 (closed))

See also #62

added Apps GMM Services labels

mentioned in merge request !220 (merged)

mentioned in commit b2a6a85f

closed with merge request !220 (merged)