Rxc density plots and parallelization cluster making

[hikari-murayama]

Thanks for opening a PR!

• [x ] Have you followed the guidelines in our Contributing Guide?
• [x ] Ensured you are not committing large data files or other sensitive information (e.g. voter files)

Please list any Issues this PR addresses

• Fixes Making clusters for parallelized functions #111
• Fixes ei_rxc() plots=T doesn't produce density plots #116

In 2-3 sentences or a brief list, describe changes implemented in this PR

• Adds a time out for parallelization (keep for now as need to update to most recent version of R to not get an error)
• Changed column names for rxc plotting

[lorenc5]

This just hung for like an hour, and produced one plot, pct_husted_pct_spiegel_hisp.png. So something is getting caught there. I ran it with par_computer=T and same thing. Thoughts?

[hikari-murayama]

I think part of the reason it's taking so long is that there are significantly more points the rxc chains have compared to ei. On the server it takes ~50 minutes to create one plot. But I'd need to take a closer look to see if anything is caught.

The plotting portion doesn't currently have a parallel option but that can definitely be implemented seeing how this takes so long.

[lorenc5]

What about the possibility that the user specify the comparison(s) they want, e.g.: pct_husted vs. pct_montanez? Instead of having the function loop through all the possible comparisons. Just a thought.