Updated static raster plots in episode 4 from base R to ggplot

[justinmillar]

Updating lesson 4 for issues #133 based on @achlandeer open PR

[jsta]

Thanks for this! Unfortunately we need the changes to be in _episodes_rmd/04-raster-calculations-in-r.Rmd not _episodes/04-raster-calculations-in-r.md. Can you port them over?

[ErinBecker]

Thanks @justinmillar for this contribution. I can take care of porting the changes over the the .rmd file in the next couple of days.

[ErinBecker]

Taking me longer than expected, as I haven't figured out how to render this lesson locally yet and need to install all the software. This is something I needed to do anyway to make sure a complete novice (me!) can follow the install instructions, so it will be a good check. Once I have everything installed and can render the lesson, I'll move these changes over to the .Rmd file. Thanks for your patience @justinmillar!

[ErinBecker]

Update - it looks like there has been another PR that makes these same changes (base plot to ggplot for episode 4) (#158). Since that PR has already been merged, I'm going to close this one.

Thanks for the contribution @justinmillar. Sorry for the confusion around this PR.