BugFix: Delete intermediate instationary stage vectors when timestep failed

When one step method throws, it may leave some memory leaks. This happens because it uses raw pointers and they are not free on the catch declaration. I would like to change all of these to shared pointers but that would mean that instationary grid operators would also have to change to shared pointers. That has to be done carefully with a proper discussion, though. So, in order to solve it as soon as possible, let's do this hotfix to resolve the issue.