updated cross validation for review

[btskinner]

Rather than hit @wdoyle42 with a bunch of suggestions, I've made and pushed a number of changes. Here's an overview:

• general reorganization so that directory paths, utility functions, and useful macros are at the top of the file
• no right hand assignment (->)
• added some utility pasting functions (not using all at the moment, so we may drop)
• added more arguments to some existing functions to remove magic numbers
• combined some {dplyr} pipes when possible for cleaner flow
• general code clean up (linting) to roughly follow the Tidyverse Style Guide
• converted $ to [["variable_name"]] to allow future flexibility (can use columns names in a variable/object with second form, but not the first)
• renamed r folder to rmd since that seems more appropriate
• added a figures directory and have the one figure saved to that
• updated .gitignore

This works when running chunk by chunk, but I did have some problems at the very end when running rmarkdown::render(). Not sure what's going on.

I'm assigning @wdoyle to review and make suggestions for me to fix. Once we've iterated, @ksalazar3 and @ozanj can review. But we can do it another way if others want.

[btskinner]

@wdoyle42: did you make some changes to the master branch version of predict_grad.rmd after I pushed my changes for the cross_validate branch version? Can you accept the cross_validate branch for merge first and then re-add your changes (or just reject my changes first)?