Missing header + something unclear in 7-Appendix/WeakPtr.ipynb

One issue and one small suggestion regarding the final code in this notebook:

• #include <algorithm> is missing (for remove_if())
• It is pretty unclear to me why, in Father::Print(), line assert(shared->IsInitialised()); is always fine, although method Child::IsInitialised() is not even declared in release mode. A simple comment in the code cell may very well settle this, but right now, it felt really confusing to me.