Correct calculation of von Mises shear strain invariant in 2d mode
The Atomic Strain modifier always uses the same formula to calculate the von Mises shear strain invariant from the components of the atomic strain tensor, even in "2d" mode. For planar systems, however, a different formula should be used, which can be found in the following paper:
(https://www.nature.com/articles/srep45550)
The current implementation in OVITO should be adapted accordingly.