Tidy slide

[ryantibs]

Implement a tidy eval version of sliding! Much more convenient, since we can do something like this:

x %>% 
  epi_slide(cases_7dav = mean(cases), n = 7)

instead of this:

x %>% 
  epi_slide(f = function(x, ...) mean(x$cases), n = 7,
            new_col_name = "cases_7dav")

Also, I refactored pct_change() and estimate_deriv() were refactored so that they just operate on vectors, rather than on epi_df objects, and they can be used inside epi_slide() fluidly. This all closes #18.

Separately, I also decided to go with the following naming scheme: any function which operates on an epi_df object starts with epi. This followed a suggestion of @earowang. So we now have:

• epi_slide()
• epi_cor()
• epi_detect_outlr()

Working in R studio, a user can therefore type epi_, then tab, and they will readily see what functions the package offers to operate on epi_df objects.

Finally, I updated the landing page (index.md) to give an overview of what the package offers.