Better truncation handling

[seabbs]

Description

This PR partly addresses #34 and some concerns in #37 about the handling of truncation. It moves to using normalisation outside the integral everywhere rather than just in the vectorised code. As @SamuelBrand1 pointed out this is more correct.

It also

• adds a variety of speed-ups to the R code to make estimation more efficient
• Improves and adds tests that compare sampling with numerical methods
• Improves coverage of pprimarycensoreddist
• Updates all documentation
• Recharaterises the Stan integral as being from delay - window to delay rather than 0 to window. Testing found this to be more stable

I investigated using xc and found this made the integral fail to solve at any tolerance level.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[seabbs]

LGTM