Revisit hypocentral depth implementations

Created by: pmpowers-usgs

Campbell and Bozorgnia (2014) use hypocentral depth. We've implemented it as (zTop - zBot) / 2. Perhaps something different along the lines of CB14's suggested models should be used. Zhao '06 also considers hypo depth but we fix it at 20km for interface sources (which sometimes isn't even in the fault plane) and use zTop for slab sources. Perhaps zHyp should be included in source XML files.