Avoid the use of raw pointers. Use std::unique_ptr instead.

[bangerth]

Inspired by #5488, these are the only places I could find where we still call operator new explicitly. This is, strictly speaking, an incompatible change because it changes the interface of a plugin system. Thoughts?

[tjhei]

This certainly breaks any plugin that contains a postprocessor of that kind. Not ideal, but this should result in a compilation error and is easy to fix. Maybe we leave a comment in the function documentation about how to fix this? If yes, I would be ok to go ahead with it (I assume we will be doing larger breaking changes going to 3.0 anyways)

[gassmoeller]

I agree, please add a note to the comment above the function and also a changelog entry (so that we can compile a list of incompatible changes for the release notes). Then this is good to go.

[bangerth]

I added the changelog entry. As for the note with the function, do you mean the declaration of the function in the base class? I don't like such notes that discuss what the interface used to be very much because they tend to be forgotten there and in a couple of years, they just describe a state that has since long been forgotten -- in other words, they become stale quite quickly. My philosophy is that the documentation should document the current state. The changelog is the right place to mention changes.