Improve discretise

[seabbs]

primarycensoreddist now supports numerically solved double censored pmf calculation (https://primarycensoreddist.epinowcast.org/dev/articles/primarycensoreddist.html#compute-the-primary-event-censored-probability-mass-function-pmf-with-dprimarycensoreddist) which we should replace our current approximate dicretise approach with either wrapped or directly.

[seabbs]

@sbfnk as discussed f2f today

[kaitejohnson]

Will open a new issue re making more explicit how the indexing is handled (starting at t=1, as indicated in model definition) in the case when a user passes in a discrete pmf.

[seabbs]

Does anyone want to take this on? I think it would be better if it wasn't me implementing given I am so in the weeds already on primarycensored.

[seabbs]

Happy to take this on at the same time as #350 and using a similar approach to epinowcast/epinowcast#549 (but using that code internally vs in a vignette/example).