GMPE subclasses without required depth parameters

[emthompson-usgs]

At least one GMPE that uses a depth parameter does not require the depth parameter in the SitesContext (e.g., BooreEtAl2014). Other GMPEs require the depth parameter (e.g., AbrahamsonEtAl2014, CampbellBozorgnia2014, ChiouYoungs2014). So I would like to request subclasses of these latter GMPEs which do not require the depth parameters. I think this would be a fairly trivial change; in AbrahamsonEtAl2014, I think it should be as simple as setting z1pt0 = z1pt0ref. Currently, we are providing a SitesContext that achieves the same result, but it requires a bit of needless bookkeeping when working with multiple GMPEs that use different equations for z1pt0. I would think that this change may be useful generally also since the depth parameters are often unavailable.