Avoid boost::optional/variant in favor of std_cxx17::variant/optional.

[bangerth]

I don't have CGAL installed, so can't test whether this actually works. But the uses of the old boost classes appear all internal to me, not in the interfaces to CGAL, and so I think this should work.

/rebuild

[fdrmrc]

This doesn't compile since the return type of CGAL::intersection is really a boost::optional<boost::variant<>>.
When I added these utilities, I used decltype(auto) (as per the CGAL doc), but in the review process we decided to be explicit about the return type.

[bangerth]

Ah, that's a bummer. How do people feel -- is it worth converting the CGAL types to std:: types for consistency throughout our code base, or should we just leave things as is?

[fdrmrc]

Thanks for cleaning this. Since CGALWrappers require C++17, you could also replace std_cxx17 with just std.

[bangerth]

Yes, it took me a number of tries to get this right, but it compiles now. It's ready from my side to be merged.

If you all would allow me to, let's leave it with std_cxx17 for the moment. We'll do a bulk replace when we will require C++17 everywhere, but for the moment it might create more confusion than help to use different conventions in different places.