Default -Wall -Werror and the skipped errors

@wizmer copied the flags from:
https://github.com/BlueBrain/hpc-coding-conventions/blob/master/cpp/cmake/bob.cmake#L192

What is the plan for choosing what is on that list and what is not?

As the compilers have become more feature-ful, and more aggressive with optimizations, my defensive programming sensibilities have kicked in even stronger. Turning on -Wall & -Werror seems like a necessity to me (with the caveat that one can choose to only enable them for non-Release builds - but the CI should run those for testing).

With that in mind, I like to try and keep the list of exceptions minimal. On that list, for instance, I think `-Wswitch-enum` is an important warning: the fact that one has to make some `switch` statements have to be complete is outweighed by missing cases.