cumerr_plot: Cumulative relative error for reaches
In markwh/rtvalidate: Validation functions for SWOT rivertile data

Plots cumulative relative error against node index, sorted by node-level relative error magnitude.

1 2	cumerr_plot(valdf, reach_ids = "all", variables = c("wse", "width", "area_total"), desc = FALSE, plot = TRUE)

`valdf`	As returned by `rt_valdata()`
`reach_ids`	Vector of `reach_id`s to include, or "all"
`variables`	Vector of variables to include, or "all"
`desc`	If TRUE, sort x-axis by descending node-level relative error
`plot`	if FALSE, return the data used to plot, but do not generate the plot.