Better stopping criterion

[Harmen Stoppels]

ARPACK uses a slightly better stopping criterion to detect convergence of a Ritz pair:

[Harmen Stoppels]

Note that ||H_m|| should be computed from the largest singular value, which is available because we already compute the eigenvalues of H_m. only when H_m is symmetric :p

[Harmen Stoppels]

I'm wondering if we can't just use is_offdiagonal_small with a lower tolerance.

[Harmen Stoppels]

Closing this for now since #57