improvements on the eigen executioners #2862
fix NonlinearEigen, this ci needs a regold #2862
a new nonlinear eigenvalue test #2862
another eigen test with a coupled system #2862
Merge pull request #2863 from YaqiWang/eigen_fix_2862

This PR fixed few things in the eigen executioners and add two new tests
Merge commit 'f06e5b31fd261fc2475f010176f9aebf291f89b8'