Add split-apply-combine explanation to dplyr episode

[lcsanford]

In previous experience teaching dplyr I have found that using Hadley Wickham's split-apply-combine language has helped students understand the power of dplyr, and might make some of the later exercises (and diagrams) easier to understand.

[lachlandeer]

@lcsanford - Thanks for the suggestion. I'll leave this as an open issue to bve worked on

[albhasan]

plyr is now a retired package. The recommended replacement package is purrr now.

Hadley Wickham's book "R for data science" has a section on purrr's map functions, which, I believe, follow the split-apply-combine pattern.

However, I think @lcsanford's suggestion fits better to a software lesson rather than an introductory data lesson on geospatial data.