14 plotting funcs

[loffleraSMH]

Closes issues #14 and closes #112

All code is on a single branch (14_plotting_funcs) but I separated the previous "plotting functions" issue into 2 separate issues that can be reviewed independently of each other (because there is a lot of code/documentation to be reviewed).

• The reviewer for #14 should review the vignette plotting_data_exploration.Rmd and the 2 functions covered in that vignette: plot_summary() and plot_over_time
• The reviewer for #112 should review the vignette plotting_theme.Rmd and the 2 functions covered in that vignette: plot_theme() and gemini_colors

A knitted version of the most up-to-date vignettes is saved here: R:\GEMINI\Research Projects\Rgemini\

I'd recommend that reviewers start by going through the vignettes and make sure the rationale & use of the functions is explained clearly. Reviewers should then review the actual function code and test the functions on additional scenarios (e.g., different tables, edge cases like missing data/weird variable types etc.).

[loffleraSMH]

@shijiaSMH: I've added some unit tests for plot_over_time(). I'll add more if the review reveals any bugs/edge case scenarios that would be worth adding specific unit tests for.

[loffleraSMH]

@shijiaSMH: I've implemented one additional enhancement for plot_summary to allow users to specify a facet_group when they only plot a single plot_var. This is useful to check distributions by individual sites (or other grouping variables). Could you take a quick look at section 3.3. in the vignette to see if that makes sense? Thanks!