Add support for liquid media (Vs==0)

[christophersanborn]

It is already possible to input Vs=0 into the model grid, and basic ray-tracing runs fine. However, the following issues exist with the scattering and Q dynamics:

• Specifying a finite, non-zero Qs via a QmQk() constructor, when Vs is zero, produces infinite Qp, and does not alert user that an illogical value was input, and a likely unintended Qp value was inferred. In this case, the user should inout Qp explicitly, rather than rely on automatic computation.

• Vs==0 results in scattering parameters el and gam0 being infinite, and MFPs for both P and S end up being NaNs. Not actually sure what effect this has during runtime. Probably effectively turns off scattering in these regions. Not a problem for S waves, since they won't be there, but for the P waves this is probably an unintended result.