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.
It's really good, the only thing I would add is a 1D, worked example of an (possibly asymmetrical, e.g. [0, -1, 1, 0]) difference filter convolution against a simple step signal. People find that very intuitive. This could be done on the board rather than in code.