Replace std::binary_function with std::function

[iondune]

std::binary_function is deprecated in C++11, removed in C++17 - replace with std::function

[brofield]

I'm happy to merge something like this, but I suspect that std::function may still not be supported in older libraries or other platforms. What have you tested this with?

[Chocobo1]

std::function is added since C++11.
So there should be a #if checking for older C++ version.

[zhmu]

Note that all that std::binary_function<> does is add a few typedef, which do not seem to be used in this code. Thus, the inheritance can safely be removed.

Note that std::function<> is a different beast, and I would not use it here (it does not add anything): all the compiler cares about is that it can call operator(), which is the case. So please, let's not make things harder than they need to be.

Should support for C++11 become mandatory in this project, I think a lambda-function would be a better way forward (but I am having the feeling @brofield does not want to drop support for pre-C++11 and that is something we should respect)

[iondune]

I have updated the PR to simply remove the inheritance from these structs - as I understand it, this will remove deprecation warnings on C++17 compilers while preserving pre-C++11 compatibility.

[brofield]

Thanks for your contribution. Merged now.