Issue #193: Swap to num_elements from size

[seabbs]

Description

This PR closes #193 by as the title swapping to num_elements in the stan code. This seems to have no detrimental impact and fixes the reported compilation edge case as shown in epinowcast/epidist#524

[Describe the changes that you made in this pull request.]

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

All modified and coverable lines are covered by tests ✅

Project coverage is 98.85%. Comparing base (be0ae95) to head (8f8e158).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #194   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          10       10           
  Lines         524      524           
=======================================
  Hits          518      518           
  Misses          6        6           

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[seabbs]

LGTM (self-review).

[seabbs]

LGTM (self-review).