Extract the stiffness or acoustic tensor from a material.
Creates a full tensor from the material's parameters in Voigt notation as a
NumPy array, even if some entries are not defined in the symmetry of the
material. These are the elliptic coefficients of the wave equation solved
by SalvusCompute.
The return of this function is not enough to reconstruct the material
fully, as it does not include the density or M0 parameters.