I have added plotting of J2 invariant of tensors for all elements.

Previously, it was only available for 2D tensors in triangles.
Now, the 'Von Mises' option plots the J2 invariant of the full 3D tensor.
The J2 invariant is the second invariant (A_ij*A_ij) of the deviator.
In practice, what is actually plotted is sqrt(1.5*A_ij*A_ij).