Issue 36: Add benchmarking of R, Stan, and fitdistdoublecens

[seabbs]

Description

This PR closes #36.

Adds touchstone based benchmarking for:

• R side functionality
• pcens_model.stan
• fitdistdoublecens()

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 99.67%. Comparing base (908dd08) to head (d3b95e3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #79   +/-   ##
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  Coverage   99.67%   99.67%           
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  Files           8        8           
  Lines         310      310           
=======================================
  Hits          309      309           
  Misses          1        1           

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[seabbs]

LGTM (self-review)