[ENH] Update datamodel with results pdf

The RealValue mixin has had pdfvariable, pdfprobability and pdfbinedges added to it, and on the IO side the real values a, b, numberevents now optionally parses these extra pdf fields

A test has been added to check this new input. There was also an update to a number in one of the test files as this was a bug.

Removed the unnecessary optional=True as this is has been made a default. It is no longer the case where 'value' must always be populated, now either or both the pdf arrays can take place here. We can introduce a check to make sure one of these fields is populated, but I didn't think it was really required.