Refactor a bit the way Docker images are used; the third party warnings are now a full-fledged demo.

Exercices about templates have been refactored (and greatly reduced: all the error handling will be tacked in the next notebook).

Move the TPs inside a same directory (thus enabling symbolic links for initial_file.cpp in each directory). Fix issues that appeared in Docker: undefined behaviour at one point and missing cassert header in another.