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

• 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #109      +/-   ##
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+ Coverage   99.35%   99.80%   +0.45%     
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  Files          10       10              
  Lines         463      510      +47     
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+ 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:

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

[seabbs]

LGTM (self-review). Checked with consistency between stan and R + numerical vs analytical in R.