Extend ppc_pit_ecdf and ppc_pit_ecdf_grouped to handle discrete data #315

The current PIT computation in ppc_pit_ecdf and ppc_pit_ecdf_grouped only works for continuous values.
Implementing randomized PIT, see, e.q., Czado et al. (2009) Eq.1, would extend the functions for discrete valued data.

At the same time, I could add axis labels and make the confidence bands dashed lines, as they are now too prominent.

• Implement randomized PIT.
• Add axis labels.
• Make confidence band lines dashed.