Include headers relative to -I

[tpadioleau]

• simplifies the logic: always relative to the root,
• should better avoid clashes with namings like "macro.hpp" or "config.hpp" that other projects could also define.

[tpadioleau]

Most of the changes are making things slightly worse. We probably should gather more documentation on how to deal with #include before committing to a choice.

Well a choice was made when we started the project. Of course we can change it if there is a better choice. Here is my understanding of the choice (note that I may have again misunderstood this choice):

• we use double quotes for internals headers to actually show they are part of the project,
• we rely on the search path of the compiler, see CMakeLists.txt the include path is injected in the build interface of the target.

[tpadioleau]

Back to draft the time we get consensus