Fix Saint-Venant Kirchhoff implementation

[FabienPean-Virtonomy]

StressCauchy is inherited from LinearElasticSolid which is an incorrect formula.

This PR fixes that and restores $\mathbf{F}\mathbf{S}\mathbf{F}^T/J = \boldsymbol\sigma$ invariant

Implementation follows https://help.febio.org/Manuals/FEBioTheory/FEBio_tm_3-0-Subsection-5.2.1.html