Added increment to level status

[brownbaerchen]

This is needed for declaring convergence based on the increment when using the polynomial based error estimate, which writes the error estimate to level.status.error_embedded_estimate, just like the increment based error estimate.
In $\Delta t$-$k$-adaptivity with RBC, I use a stopping criterion that uses both residual and increment, so I need both the error estimate and the increment.
This PR should be seen as a bugfix. Previous tests for CheckConvergence are still valid.