1009: don't round for PIT histograms

[sbfnk]

Description

This PR closes #1009.

Whilst there also fixes another bug later on.

Checklist

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[nikosbosse]

@sbfnk could you give a slightly more detailed description of the bug? If I understand correctly

• we incorrectly always computed the discrete version of the histogram
• we rounded numbers inappropriately
  Is that correct?

[sbfnk]

Yes, exactly. Not quite sure where the rounding came from but it might be a leftover from inserting a statement for testing.