fix a logic error: an eigen kernel should not mark its variable as eigen variable when eigen==false  #2902
this ci seems changing nothing but actually enables the k being set by Executioner when EigenKernel's eigen is false (important to nonlinear diffusion acceleration in neutronics) #2902
a cleaner implementation, and also allows doing on-the-fly fix instead of throwing errors #2902
Merge pull request #2903 from YaqiWang/eigen_2902

Improve eigen system 2902