177: use num_elements instead of size

[sbfnk]

Description

This PR closes #177 by replacing size with num_elements.

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.

[seabbs]

As noted in the issue I am quite unclear why this didn't work for you in the first place given it works on CI and in epidist. @athowes did you have some m3 issues and did it look the same? Any comment on this

As this is causing issues regardless of why I am happy to change but can you first update the news file and add yourself as a contributor in the description

[athowes]

Here is the error I had: epinowcast/epidist#221 (comment).

It was to do with stan::math::size so I think it might be the same but unsure.

[sbfnk]

Yes, I think that was the same error - the issue being the failing implicit conversion from size_t to int, see also stan-dev/math#3085

size_t is platform specific so I think this might explain why it fails only on some systems

[seabbs]

I think there isn't a point to forcing people to use a different compiler flag if this is all Mn MacBooks which it sounds like it is so this change to the code makes sense if it works for both of you (and as it works everywhere the old version worked).

[sbfnk]

This may also be fixed as of yesterday
https://github.com/stan-dev/math/blob/9bae49db12815e6906765788663ff6b6f04769fc/RELEASE-NOTES.txt#L27

[seabbs]

My preference would be to keep using size as its shorter and so makes the code easier to parse but its a weak preference. Perhaps we merge this and I will make a issue to revert once the things flows out and is confirmed

[sbfnk]

add yourself as a contributor in the description

turns out I'm there already

[seabbs]

😱

[seabbs]

LGTM

[sbfnk]

I still get (almost) the same error with the latest stan math library -- size_t has been replaced by int64_t but conversion to int still yields a warning.