Allow existing uses of _jacobian in function names, with warning

[WardBrian]

Closes #1470. This uses the basic rule that if the functions block contains no uses of the new jacobian += statement, it is probably older code, so we should keep allowing it for now.

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Fixed a backwards-compatiblity hazard with the new _jacobian functions. Existing functions that just happen to have this naming structure will no longer fail to compile, but will issue a warning alerting that this name will change meanings in the future.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[nhuurre]

Looks good.

In unrelated news, the following error is not detected until C++ compilation:

functions {
  // void function
  void foo_jacobian() {
    jacobian += 1;
  }
}
transformed data {
  foo_jacobian();
}

[WardBrian]

@nhuurre good catch! so did this:

functions {
    void foo_rng(real x){
        print(normal_rng(0,x));
    }
}

model {
    foo_rng(1.0);
}

Fixed both.

[nhuurre]

void rngs are useless, but I suppose there's no harm in allowing them.

[codecov]

Codecov Report

Attention: Patch coverage is 66.00000% with 17 lines in your changes missing coverage. Please review.

Project coverage is 89.67%. Comparing base (96e409a) to head (02c8663).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/frontend/Deprecation_analysis.ml	62.22%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1471      +/-   ##
==========================================
- Coverage   89.76%   89.67%   -0.10%     
==========================================
  Files          63       63              
  Lines       10613    10656      +43     
==========================================
+ Hits         9527     9556      +29     
- Misses       1086     1100      +14     

Files with missing lines	Coverage Δ
src/frontend/Typechecker.ml	93.05% <100.00%> (+0.27%)	⬆️
src/middle/Fun_kind.ml	90.90% <100.00%> (ø)
src/frontend/Deprecation_analysis.ml	72.63% <62.22%> (-9.37%)	⬇️