Issue 85: Add infrastructure for analytical solutions in R

[seabbs]

Description

This PR partially closes #85 by adding infrastructure in R for introducing analytical solutions. It also adds skeletons for the current solutions but does not add the analytical solutions themselves.

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.
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[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 6 lines in your changes missing coverage. Please review.

Project coverage is 98.06%. Comparing base (5c23d7a) to head (9a48117).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
R/primary_censored_dist.R	86.04%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   99.67%   98.06%   -1.62%     
==========================================
  Files           8       10       +2     
  Lines         310      362      +52     
==========================================
+ Hits          309      355      +46     
- Misses          1        7       +6     

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[seabbs]

I'm fairly horrified by what I had to do here to get this working but despite that I think its the most sensible approach that retains modularity

[seabbs]

LGTM (self-review)

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 596c03f is merged into main:

• ✔️cmdstan_fit_gamma: 7.7s -> 7.75s [-0.23%, +1.69%]
• ✔️cmdstan_fit_lognormal: 1.65s -> 1.63s [-5.54%, +3.22%]
• ❗🐌dprimarycensoreddist_expgrowth: 1.95ms -> 2.34ms [+17.38%, +22.82%]
• ❗🐌dprimarycensoreddist_weibull: 1.87ms -> 2.27ms [+18.7%, +24.27%]
• ❗🐌fitdistdoublecens_gamma: 665ms -> 678ms [+0.92%, +3.01%]
• ❗🐌fitdistdoublecens_normal: 557ms -> 562ms [+0.65%, +1.47%]
• ❗🐌pprimarycensoreddist_expgrowth: 37.5ms -> 45.4ms [+19.21%, +23.35%]
• ❗🐌pprimarycensoreddist_lnorm: 34.4ms -> 42.2ms [+20.52%, +25.38%]
  Further explanation regarding interpretation and methodology can be found in the documentation of touchstone.

[seabbs]

Test gaps are due to the current lack of analytical solutions so I think this is good to go.