refactor stu_RIT_hist_plot_elements to take mapvizieR object

[almartin82]

@chrishaid I think this is some low-hanging fruit that would help you get up to speed on the new plots that I am bringing in.

There is a function called stu_RIT_hist_plot_elements

that does some common calculations and generates some of the ggplot objects used when making student college plots. This has been ported over from a pre-mapvizieR script, so it's not aware of the mapvizieR object.

right now it takes a data frame with a student's RIT history (I think cdf-style); the mapvizieR behavior would be to take a mapvizieR object and a studentid, and then have the function yank out the relevant rows internally. do you want to take a crack at this? you'd need to:

• [ ] refactor the parameters to take mapvizieR_obj and studentid
• [ ] extract the cdf and limit at the top of the function
• [ ] look for references to where stu_RIT_hist_plot_elements is used and fix those calls.

I can also take this, but I was on the lookout for something that would be a good head-first introduction into this code, and this came to mind.