Revamp the TP for procedural programming:  make it clearer, add one session and put some exercices in a later one to avoid a heavy block with more then 10 exercices at once. Solutions have been rewritten to be generally more elegant; I have also renounced to separate declaration and definition as this was definitely a pain for most participants the first day.

Fix a plain mistake: overloads of operator<< should not be in namespace std! Rewrite the streams notebook appropriately; unfortunately doing so render the example code not runnable on Xeus-cling. I have added explicit instantiation of operator>>. In the namespace notebook, I have added a part about namespace std and another one to say where overloaded operators should be added.

Insist on the = 0 for pure virtual case, and underline it is nonetheless possible to provide an implementation for a pure virtual method.

Remove executed cells from git: we should strive to commit notebooks which kernels have just been cleared.