Add version check when exposing external functions

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

This PR closes #713. Currently expose_functions() fails with an uninformative error message if external c++ is used. This is due to a bug in stanc3, and after this PR the generated code will be compatible.

This PR adds a simple version check when exposing external functions

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

Looks good, thanks @andrjohns!

[codecov-commenter]

Codecov Report

Merging #747 (af1954a) into master (4c4dd09) will increase coverage by 2.16%.
The diff coverage is 100.00%.

❗ Current head af1954a differs from pull request most recent head 3642755. Consider uploading reports for the commit 3642755 to get more accurate results

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   86.20%   88.37%   +2.16%     
==========================================
  Files          12       12              
  Lines        4082     4086       +4     
==========================================
+ Hits         3519     3611      +92     
+ Misses        563      475      -88     

Impacted Files	Coverage Δ
R/model.R	90.56% <100.00%> (+0.14%)	⬆️
R/utils.R	74.51% <100.00%> (+2.10%)	⬆️

... and 3 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more