Seesaw

Hi @rkneufeld, all,

Please find here my PR for an example Seesaw recipe, a greatly-revised version of the original Seesaw PR from @JouisWalker.

I have placed several small screenshots throughout the text.  These do not really merit separate figures by themselves, but could be merely shown inline as is, if O'Reilly are OK with that approach.

I attempted to address the fact that this is just one example of the many things which are possible with Seesaw.  My concluding remarks could be amended slightly if someone produces several additional Seesaw recipe.