review timeProp()

[jack-davison]

This PR fixes #438.

timeProp() had a few random odds and sods in it that weren't being used, had some repetition, and a bit of "round-the-houses" ways of achieving certain things (e.g., colours). This PR simplifies and modernises a lot of this.

One obvious user-facing feature is the addition of key and rejigging the argument order to be more in line with timePlot(), which it now inherits a lot of its param definitions from.

A more subtle one is that proportion is now passed to type in checkPrep() rather than as a member of vars. This means you can pass "default" to it, which means you can create a bar chart out of basically any statistic as long as avg.time is the same as what you're starting with! This fixes the spirit of #122 via something like:

mydata |>
  selectByDate(year = 2000) |>
  rollingMean() |>
  timeAverage(avg.time = "day", statistic = "max") |>
  timeProp(
    pollutant = "rolling8o3",
    proportion = "default",
    avg.time = "day",
    sub = NA,
    key = FALSE
  )