MS_POISSON - Poisson's ratio in anisotropic media.

Calculate Poisson's ration from an elsticity matrix.

nu = MS_poisson( C, m, n )

Usage:
    nu = MS_poisson( C, m, n )
        Calculate Poisson's ration for anisotropic media given an
        elasticity matrix, C, longitudinal direction described by
        a unit vector, n, and transverse direction given by the
        unit vector m.

Notes:
   Poisson's ratio, nu, is normally described as the negative ratio of
   a (typically) compressive strain normal a direction of extensional
   strain. In an isotropic material nu takes a single value between -1.0
   an 0.5 (positive values around 0.2 or 0.3 are common). For the general
   anisotropic case Poisson's ratio depends on the longitudinal extension
   direction and the direction perpendicular to this (the transverse
   direction) where the compression (positive nu) or extension (negative
   nu) is measured. This function allows nu to be calculated as a function
   of these two directions, given as unit vectors. The approach follows
   equation 3 of Wojciechowski (2005)

Reference: Wojciechowski, K. W. (2005) "Poisson's ratio of anisotropic
    systems" COMPUTATIONAL METHODS IN SCIENCE AND TECHNOLOGY 11, 73-79.

See also: MS_PHASEVELS