PK2 stress in post processing is not symmetric

[h-vijayakumaran]

This is in reference to the hyperelastic RVE case 9ab1931, but the issue could be present in other cases as well. In the post processing step, we retrieve the applied deformation gradient $\bar{\mathbf{F}}$ and obtain the homogenized 1st Piola-Kirchhoff stress $\bar{\mathbf{P}}$. However, when I compute the 2nd Piola-Kirchhoff stress $\bar{\mathbf{S}} \coloneqq \bar{\mathbf{F}}^{-1}\bar{\mathbf{P}} $, I get a non-symmetric matrix, which shouldn't be the case. There seems to be a bug in the post processing and this needs to be investigated and resolved.

[JiaxiangYi96]

Hey, Hari. Let's work together to resolve this issue later :D