Fix an error in the calculation of the stress eigenvectors. The strain tensor contained a wrong summation, which has been removed. Additionally, the stress tensor did not contain the full strain trace on its diagonal. Both issues have been fixed.
Add the configuration option continuous to the stress EV visualization. This allows toggling of a P1 (vertex) or a P0 (cell) representation of the EVs.
Remove output of the index of the largest EV. This simply was leftover code from !8 (merged).
