Initial commit for packrat installation in R

[manasvita]

Initial commit for the instructions on using Packrat in R

[manasvita]

@pkrastev, @pedmon, & @paulasanematsu, assigning you as reviewers here. Please let me know if the instructions look good and if this is an appropriate place for the document.

https://github.com/fasrc/User_Codes/blob/packrat-mj/Languages/R/Packrat_for_R.md

[manasvita]

Hey @paulasanematsu, could you take a look at this and let me know if any edits are required or not? Will go ahead and merge the branch otherwise. Thanks.

[paulasanematsu]

I successfully installed packrat and viridis following the document instructions.

You probably need to be clear which RStudio these instructions are valid for. They would work with Remote Desktop -> launch RStudio Desktop from within Remote Desktop. However, they will not work on RStudio Server app because you cannot set R_LIBS_USER and you cannot load modules either.

I am not entirely sure, I would have to discuss with Nathan Weeks to be sure, but packrat may not be a good solution for RStudio Server because Nathan uses locked-in versions of packages to ensure package compatibility.

[paulasanematsu]

Also, you might want to include steps to use packrat in two more instances:

1. after user logs out. How do you get back to the same packrat environment?
2. in a job script. How do you use a packrat environment in a batch job?