Extend ppc_pit_ecdf and ppc_pit_ecdf_grouped to handle discrete data

[TeemuSailynoja]

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.

[TeemuSailynoja]

I have this mostly done, as I have needed this functionality with @avehtari in a few occasions, so I would just need to add it to a PR.

[jgabry]

Closing now that #316 was merged. Or are there still things to do? If so we can reopen.