[CIG-SEISMO] SPECFEM3D Tomography Model

[Yingzi Ying]

Dear SPECFEM Developer,

As we know, in SPECFEM2D an acoustic spectral element can be defined 
just by setting vs to be zero.

In my opinion, it will be very useful if we can also define an acoustic 
element if the vs value read from "tomography_model.xyz" is zero in 
using tomography model in SPECFEM3D.

I don't know if it is difficulty to realize it by modifying the code in 
"get_model.f90" or some where else.

Hope this function will be realized in the near future.

Best regards,
Yingzi