Replace for loops with map function

[seanofahey]

The use of loops in the code leads to poor performance for large surveys. Performance can be improved using the map function throughout the code.

[sfirke]

Great call. I started reworking my fork of the package along these lines b/c when things went wrong, the nested loops were impossible to debug. Let me see if I have anything that will be usable for you.

[sfirke]

If you look at the diff of my fork at sfirke/rmonkey vs this package, you'll see my start of this. E.g., this commit: sfirke@2926512 shows some modular functions that are designed to call each other with purrr::map().