Issue 85: Implement analytical solutions in R

[seabbs]

Description

This PR partially addresses #85 in that it adds analytical solutions in R for the gamma and lognormal with uniform primary censoring.

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 99.76%. Comparing base (11d0c7f) to head (006d118).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   98.06%   99.76%   +1.69%     
==========================================
  Files          10       10              
  Lines         362      423      +61     
==========================================
+ Hits          355      422      +67     
+ Misses          7        1       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 006d118 is merged into main:

• ✔️cmdstan_fit_gamma: 7.64s -> 7.66s [-0.43%, +0.88%]
• 🚀cmdstan_fit_lognormal: 1.65s -> 1.57s [-8.28%, -0.54%]
• ✔️dprimarycensoreddist_pweibull_expgrowth: 2.35ms -> 2.35ms [-1.34%, +1.72%]
• ✔️dprimarycensoreddist_weibull: 2.32ms -> 2.3ms [-2.2%, +0.53%]
• 🚀fitdistdoublecens_gamma_expgrowth: 667ms -> 661ms [-1.35%, -0.55%]
• 🚀fitdistdoublecens_normal: 553ms -> 549ms [-1.02%, -0.51%]
• 🚀pprimarycensoreddist_lnorm: 44.7ms -> 1.9ms [-96.87%, -94.64%]
• ✔️pprimarycensoreddist_plnrom_expgrowth: 47.7ms -> 47.7ms [-1.06%, +1.14%]
  Further explanation regarding interpretation and methodology can be found in the documentation of touchstone.

[seabbs]

LGTM (self-review)