913: Functions for PIT histograms

[sbfnk]

Description

This PR closes #913.

It adds plot_histogram_sample() and get_pit_histogram() functions and implements the nonrandomised PIT for count data suggested by Czado et al.

It also removes get_pit() and pit_sample() - I don't think calculating PIT values themselves is very interesting or something anyone would ever want to do but if people disagree we could keep them in.

It also removes plot_pit(). Now that densities of the histogram can be obtained directly I would think that a user will be able to make their own plot.

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