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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.51%. Comparing base (74c2c8c) to head (5727045). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #44 +/- ## ========================================== + Coverage 99.48% 99.51% +0.03% ========================================== Files 6 6 Lines 194 207 +13 ========================================== + Hits 193 206 +13 Misses 1 1 ```

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