Add ECDF plot and ECDF difference plot with simultaneus confidence intervals as graphical goodness-of-fit and convergence diagnostics

As requested by @avehtari and @hyunjimoon, implement the ecdf plots from [this article](https://arxiv.org/abs/2103.10522). Code for the article is available [here](https://github.com/TeemuSailynoja/simultaneous-confidence-bands/blob/main/ecdf_plot_functions.R), so mostly cleaning and formatting needed.
