Add 1500 to vs30 selection options

1500 m/s is the current building code 'Site Class A' value but is absent from the list of options.

@pmpowers

Currently the drop down has site class AB with Vs30 1500. This comes from the usage.

Clarification: this issue is referring to the Site & Basin options in the three GMM applications when Site Class or Vs30 is the Multi Selectable Parameter.

In writing the above comment, I noted that the Multi-Selectable Parameter options are inconsistent. Let's use the ones in the spectra app Ground Motion Model, Magnitude, and Site Class and replace the use of Vs30 and Mw in the distance and magnitude GMM apps.

mentioned in merge request !466 (merged)

mentioned in commit b858c37d

closed with merge request !466 (merged)