Bugfixes in .stanfunctions, hessian model method, and exposing RNG functions

[andrjohns]

Submission Checklist

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

Summary

This PR comprises two small bugfixes:

• .stanfunctions was not treated as a valid extension
• math/mix.hpp was not included early enough, resulting in compilation errors for some hessian models

And one slightly larger bugfix in the handling of exposing RNG functions. Currently, we expose RNG functions with an integer seed argument (default = 0) - such that users need to change this seed argument if they want a different value on each call. This PR changes that behaviour to initialise a Boost RNG and store it in the functions environment, which is then passed to the exposed RNG function - allowing the exposed RNG function to act as expected.

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]

This PR changes that behaviour to initialise a Boost RNG and store it in the functions environment, which is then passed to the exposed RNG function - allowing the exposed RNG function to act as expected.

Good idea, thanks!

[codecov-commenter]

Codecov Report

Merging #811 (676f44a) into master (74f39dc) will decrease coverage by 1.67%.
Report is 16 commits behind head on master.
The diff coverage is 95.65%.

❗ Current head 676f44a differs from pull request most recent head 7fb88b6. Consider uploading reports for the commit 7fb88b6 to get more accurate results

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
- Coverage   88.24%   86.58%   -1.67%     
==========================================
  Files          12       12              
  Lines        4161     4175      +14     
==========================================
- Hits         3672     3615      -57     
- Misses        489      560      +71     

Files Changed	Coverage Δ
R/fit.R	96.22% <ø> (ø)
R/utils.R	72.43% <94.73%> (-1.28%)	⬇️
R/example.R	97.56% <100.00%> (ø)
R/model.R	90.46% <100.00%> (-0.14%)	⬇️

... and 3 files with indirect coverage changes

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[rok-cesnovar]

Andrew is on a roll, I love it :)

[jgabry]

Thanks @andrjohns!