Check header self-containment with CMake

[denniskb]

Add a custom CMake target that attempts to compile each header individually (by including it in a single, empty .cpp file) to make sure our headers are self-contained.

• https://stackoverflow.com/questions/59716373/compile-headers-for-errors-using-cmake
• https://stackoverflow.com/questions/16429376/how-can-i-let-cmake-check-whether-my-headers-are-self-sufficient

[denniskb]

This has very little utility as most of our classes are templated. Templates will not be checked unless instantiated.