mgxs.h5 eigenvalue 100 20 15 -0.63 -0.63 -1 0.63 0.63 1 true multi-group -0.63 -0.63 -1.0 0.63 0.63 1.0 30.0 150.0 linear 0.5 2 2 -0.63 -0.63 0.63 0.63