MS_CIJKL2CIJ - Convert from elastic tensor to Voigt elasticity matrix

Converts between a 3x3x3x3 tensor representation and a 6x6 Voigt
    representation of anisotropic elasticity.

[C] = MS_cijkl2cij(CC)

Usage:
    CC must be a rank 4 array with size (3,3,3,3). The
    returned array C will be rank 2 with size (6,6). The elasticity
    matrix representation is used as input for almost all MSAT functions.

Notes:
    Do not use this function for the elastic compliance as additional
    terms are needed in this case.

See also: MS_cij2cijkl