Rearrange contents in chapter 8

[webartifex]

https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/08_mfr/01_content.ipynb

Move parts on list comprehensions into chapter 7 where list constructors are covered (or after the part on list methods because of .append()).

Move the nested and Cartesian product examples into exercises on their own.

Convert "Example: Averaging all even Numbers in a List (revisited)" section into a new section on "Streaming data".

Move tuple comprehensions part up into generator section.

Create new section on generator functions (make a unified generator section with expression and function as sub-section).
Note that the numeric example is not the same as in the first content file.

[webartifex]

Include Fibonacci example for generators

Advantage: instead of single value, calculate entire Fibonacci sequence. Only works with iterative version.