Sketch out first 3 sections

These sections were originally IPython notebooks that I converted to markdown. This is missing a lot of explanatory text, but that will come later. I'm really not sure whether this is the right process for version control. I suspect, that each build will create new images, but I guess the text diffs are manageable (compared to diffs of ipynb files) so that's a definite plus.

Note that I created a separate file for plot helpers. I don't like having notebooks cluttered with code that isn't relevant to the topic. I plan on discussing the helpers, but just having them in a separate file minimizes distraction.
Actually, a link to the code wouldn't be preferable: It's a mess. I planned on talking about it at a high level, but it's probably a good idea to add a description of this in the notes.

It's a little more complicated than the local max, since you don't want edge pixels masking out pixels that are on the same edge. It's basically a maximum mask in the direction normal to the edge. I don't know the algorithm for the medial-axis transform, but I've only seen implementations that work on binary images, not grayscale.