J0 is never updated from DDSDDE

[sswan]

Looking at core/matmodlab.py, it seems that J0 is initially evaluated during startup but never updated (with a numerical jacobian or with the returned jacobian). Also curious, the returned DDSDDE seems to always be discarded (whether it is None or a full matrix).

[tjfulle]

I'm not looking at the code, but if I recall J0 is just the initial guess for the Jacobian and is only used to estimate the initial deformation rate. DDSDDE is not used outside of the Newton silver for strain rate.