Presentation 1: improve section 1 and 2, and comment on section 2

* Since name clash occurred for $N$, which previously stood for dimension of projective space ($\P^N$) and now also stands for the length of exact cycles ($N$-cycles), I changed the dimension to $n$.

* Robin, I reformatted your code to use 2 spaces for indenting and 70 columns per line. Tabs are evil and are usually deprecated these days (sure there are tab maniacs like Linus Torvalds, but they are the minority). On the other hand, 70 columns per line increases readability on a wider range of text editors.

    Please feel free to talk to me if you have any objection to the style. We can easily revert or reformat.

* There are some issues with section 2 (Morton N=4). See issue #3 and my comments in 87fc520.

---

By the way, sorry for my delay on the $N=5$ case. I'll finish it soon.