Handle additional intensity measures in GMM apps

It would be helpful to be able to show intensity measures other than PGA and spectral accelerations in the various GMM apps. Currently the GMvR service will return results for PGV but this does not propagate through to the apps. Addressing this issue will probably require rethinking a bit about how results are returned from services. For example, PGA is currently encoded as 0.001 in the response from the spectra service but it would make more sense to have a separate response element for PGA and this would simplify adding PGV and other response elements.