Add support for vertical ground motions

Start with pure V/H based model Gülerce & Abrahamson (2011)

• @nkwong Determine which GMMs currently in use have coefficient tables, modified functional forms, or ground motion tables for vertical
• Implement container class for Gülerce & Abrahamson (2011): V/H GM
• Add config flag "vertical": false (default)

V/H models for consideration:

• Gulerce and Abrahamson 2011 from NGAW1
• Bommer et al 2011 for Europe and Middle East
• Akkar and Sandikkaya 2014 for Europe
• Bozorgnia and Campbell 2016 from NGAW2
• Haji-Soltani et al 2017 for CEUS