[WIP] add ms_piechart

[JanMarvin]

Work in progress to add a piechart #55. Originally I wanted to add box and whisker plots and histograms, but these look like they are x14 functions. Therefore I ended up with the nasty pie chart (don't tell anyone I created this function, I have a name to lose).

library(officer)
library(mschart)

pie_plot <- ms_piechart(
  data = iris,
  x = "Species",
  y = "Sepal.Length",
  group = "Species",
  label = "Sepal.Length"
)

doc <- read_pptx()
doc <- add_slide(doc, layout = "Title and Content", master = "Office Theme")
doc <- ph_with(doc, value = pie_plot, location = ph_location_fullsize())
print(doc, target = "example.pptx")

The function is a bit ugly to be honest. If I'm not mistaken, pie charts behave a little different. Every cell of a column is piece in the pie. Therefore I had to construct a function to calculate some unique values. Currently I simply calculate group wise averages

[davidgohel]

Hello @JanMarvin

It's obvious I'm having trouble finding time to maintain mschart. @eli-daniels will handle that maintenance now. He needs some time to get up to speed with my help. I am sorry your PR are still not accepted but Eli will help with that too :)