== MS_iso

   MS_iso - generate elastic matrix from isotropic velocities

   [C]=MS_iso(vp,vs,rh)

   Inputs: 
       rh  : Density (kg/m3) 
       vp  : p-wave velocity (km/s) 
       vs  : s-wave velocity (km/s)

   Output:
        C : Stiffness tensor (6x6 notation, GPa)
