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Notebook number 12 focuses on aspects like specialisation and partial
specialisation of template classes, why specialisation of function
templates should be avoided and pitfalls of inheritance in combination
with template classes.

Commit adds examples on using the override and final specifiers on virtual
methods and an example on pure virtual functions and abstract classes. We
also improve the sectioning in the notebook.

Additionally commit fixes a typo in notebook #08.

This update now really contains the new section on std::function.

- In our VectorClass we have, of course, access to vec_ data member of
  other objects of the class; can use that in the add() and innerProduct()
  methods.
- Ran through second half of notebook #6 in class, so some small updates
  and extensions.