Readability update

Created by: anderslindho

A first (quite significant) cleanup of e3training done. Note that there still is much left to do, but better to do this in steps so we can have something more digestible up ASAP. The blunt of the work has been on rewriting the initial content so that it simply is more easily readable. The main modifications outside of this are on removing a chapter that dealt with applications (merging it with modules), and adding a chapter on how to use (some) supplementary tools.

Note that:

• I decided to keep using e3TemplateGenerator (over cookiecutter) for the e3 module creation chapter for primarily two reasons; to make it easier for myself to rewrite it (as that chapter already was a fairly large effort), and because it still is the "established" tool that people use and are comfortable with. I think it is an additional benefit that "it works out of the box" (no need for python3 and possibly venvs).
• I kept versions in already existing exercises as they were because it was easiest to ensure they still work that way.

I strongly recommend not comparing diffs but rather reading the tutorial series side by side.