Hotfix: Will correct rotation matrix for mz=-1.

tetrax/core/fem_core/rotation_mat.c

@@ -124,7 +124,7 @@ double rotation_matrix_component_nonorm(double m[3], int idx_m, int idx_n) {
     // so the rotation matrix becomes the identity operator
     if (abs_sq == 0) {
 //        printf("Warning: magnetization in ez-direction\n");
-        return idx_m == idx_n ? 1 : 0;
+        return idx_m == idx_n ? m[2] : 0;
     }
 
     switch (idx_m) {