Comprehensions

[pnhoffmann]

• add a point to list comprehensions showing how comprehensions can be nested (http://fsr.github.io/python-lessons/exercises/13_comprehension.html -> cartesian product)
• the generator chapter of presentation 6 (https://github.com/fsr/python-lessons/blob/master/latex/slides/06_comprehensions.tex) seems incomplete, it's definitely missing an example

[h4llow3En]

@pnhoffmann you are able to make a PR