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.

In the notebook we demonstrate overloading of free functions and discuss
the question of a function's signature.