Integrating and documenting new features from Ginkgo 1.6 into benchmark

[stanisic]

Congrats on Ginkgo 1.6 release! We can see quite a few new features in it, for us most notably GCR and Flexible GMRES solvers, however, the benchmarks documentation didnt change much. It appears to me that some new features (like 2 aforementioned solvers) are not yet integrated into benchmarks, while some other features are integrated but not documented.

I understand that this doesnt have the highest priority, but when time allows it would be nice if these features are available in benchmarks and briefly explained in the docs :)