Add convergence to the monitor

[timfelle]

Adds a convergence flag to the krylov monitor.
Additionally use the flag for throwing an error instead of the nan check.

[njansson]

I think all the checks should be safe false when either is NaN, so it should work. But would be good to get a second opinion on that @pohm01?

Also, before we merge the sx solvers needed to be modified as well

[timfelle]

I think all the checks should be safe false when either is NaN, so it should work. But would be good to get a second opinion on that @pohm01?

I added it as a new check instead. So now the solver reports "divergence" under NaN check as before and reports "non-convergence" for the new check.

Also, before we merge the sx solvers needed to be modified as well

Done.

[MartinKarp]

Looks good! Mostly an idea regarding the check to make future changes easier.