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.
I agree to an extent, but there are a number of fiddly bits that I'd like to avoid. I'll actually work through (or have the students work through) most of the guts of plotting images and histograms, but there's a bit of plot tweaking that I want to do that would serve as a distraction, since the point of our tutorial is to teach image processing. I definitely want to show students how to plot the images that they're generating, but it's not important to dwell on some of the details.