Lanczos parameters and status

[manuschneider]

The parameters of the Lanczos solver are explained roughly in the API documentation, but it is not clearly defined what the parameter CvgCrit exactly is (the algorithm stops if the energy difference between the previous and current iteration is below this threshold?), and what differs between the method options Gnd and ER.

Also, would it be possible to return some convergence information, such as the residual?

Currently it is difficult to use Lanczos in a well-controlled way.