ggplot-ing episode 02

[lachlandeer]

This extends the ggplot-izing I started with #147 into episode 2, inline with goals outlined in #133

I adopted the framework suggested in #155 for rendering hillshades

• All plots constructed with ggplot
• Uses dplyr for any data manipulation inside a data.frame() object

[jsta]

Looks good to me. I have some of the same suggestions as in #147 (comment). Otherwise, the only thing I see is that we still have a few lines of text that refer to baseplotting with the plot() function.

[lwasser]

so what is left to accept this PR @jsta

[ErinBecker]

Thanks for the review @jsta, I'm going to go ahead and merge this under the understanding that, even if not quite complete, this PR is a significant improvement to the episode. Let's make any additional changes in a subsequent PR, to avoid this PR becoming outdated as new contributions come in.