Issue 94: Add R and Stan for analytical Weibull

[seabbs]

Description

This PR closes #94 and adds:

• Translation of the analytical Weibull with uniform solution to R and stan
• Monte Carlo testing of the R analytical solution
• Equivalence testing of the stan and R implementations

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 99.80%. Comparing base (b84867c) to head (60a2960). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #109 +/- ## ========================================== + Coverage 99.35% 99.80% +0.45% ========================================== Files 10 10 Lines 463 510 +47 ========================================== + Hits 460 509 +49 + Misses 3 1 -2 ```

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[seabbs]

I think this is all working as expected now.

I need to do the following:

• [x] Open an issue to investigate the analytical solution in R failing when pwindow is large due to the lower incomplete gamma
• [x] Open an issue to investigate the sporadic NANs in the stan analytical solutions (all dists) when the cdf is 1.