plot_change: Plot Cover Change
In crumplecup/riparian: Analyzing the Extent of Riparian Cover

Prints a graph of cover change.

plot_change(
  csv,
  year1,
  year2,
  title = "cover_change.png",
  heading = " ",
  leg_pos = "topleft",
  y_lab = "Relative Frequency of Samples at X%"
)

`csv`	is a csv with cols (id, year, type, results(1:50))
`year1`	is an integer matching a value in csv$year
`year2`	is an integer matching a value in csv$year
`title`	is the character vector to name the output file (png)
`heading`	is the character vector used as the main title of the plot
`leg_pos`	is the legend position argument (character)
`y_lab`	is the label for the y axis (character)