Add Anderson's (1961) TI parameters

MS_TI now also does this:

   [C]=MS_TI(Vpv, Vph, Vp45, Vsv, Vsh, rho, 'anderson')

   Inputs:
        Vpv    : Velocity of P-wave along 3-axis
        Vph    : Velocity of P-wave normal to 3-axis
        Vp45   : Velocity of P-wave at 45 degrees to 3-axis
        Vsv    : Velocity of S-wave along 3-axis (or normal to 3-axis
                 polarised normal to 1-2 plane)
        Vsh    : Velocity of S-wave normal 3-axis polarised
                 parallel to 1-2 plane
        rho    : Density

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

   Calculates the elastic tensor for a VTI medium from Anderson's (1961)
   parameters. Velocities in km/s, density in kg/m^3.

   Anderson, D. L. (1961) "Elastic wave propagation in layered
    anisotropic media" Journal of Geophysical Research 66:2953 - 2963
Merge branch 'master' of github.com:andreww/MSAT