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)