MS_BUILD_ISOTROPIC - Create elasticity matrix from pairs of isotropic moduli.

Given two isotropic moduli create an elasticity matrix for an isotropic
    material and optionally return all other moduli.

[C, ...]=MS_build_isotropic('modname1', mod1, 'modname2', mod2')

Usage:
    [C]=MS_build_isotropic('modname1', mod1, 'modname2', mod2')
         Given two isotropic moduli mod1 and mod2 named as strings
         'modname1' and 'modname2' return an elasticity matrix C.
         Permitted moduli are:
             'lam' - first lame parameter, in GPa
             'mu' - shear modulus (second Lame parameter), in GPa
             'K' - bulk modulus, in GPa.
             'E' - Young's modulus, in GPa.
             'nu' - Poisson's ratio, dimensionless.
             'M' - 'P-wave modulus', in GPa.

[C, K, E, lam, mu, nu, M]=MS_build_isotropic('modname1', mod1, 'modname2', mod2')
     Also return all 6 moduli.

Notes:
    Not all combinations of moduli are valid / implemented

References:
    Stacey F. D. and Davis, P. M. (2008) Physics of the Earth 4th ed.
    Appendix D and Section 10.2.

See also: MS_POLYAVERAGE