71 feature request r based user defined functions

[mattfidler]

@john-harrold as requested.

This calls a R function inside of rxode2; Still needs

• function query on solve to make sure the functions are still accessible and have the same number of arguments
• Sane runtime warnings when the function doesn't do the expected thing (return a single value that can be changed to a double without errors)

This is likely quite slow because it travels between R and C quite a bit, though it could be convenient

[codecov]

Codecov Report

Attention: 152 lines in your changes are missing coverage. Please review.

Comparison is base (36dd96c) 50.95% compared to head (587f16d) 50.90%.

❗ Current head 587f16d differs from pull request most recent head be7213c. Consider uploading reports for the commit be7213c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
- Coverage   50.95%   50.90%   -0.06%     
==========================================
  Files          57       59       +2     
  Lines       12123    12408     +285     
==========================================
+ Hits         6177     6316     +139     
- Misses       5946     6092     +146     

Files	Coverage Δ
src/codegen.c	70.04% <100.00%> (+0.26%)	⬆️
src/etTran.cpp	72.68% <100.00%> (-0.02%)	⬇️
src/genModelVars.c	100.00% <100.00%> (ø)
src/init.c	67.30% <100.00%> (+0.64%)	⬆️
src/tran.c	87.59% <100.00%> (+0.03%)	⬆️
src/tran.h	98.59% <ø> (ø)
R/tran.R	82.66% <50.00%> (+0.23%)	⬆️
src/parseFuns.h	66.66% <87.09%> (+3.39%)	⬆️
src/udf.cpp	47.50% <47.50%> (ø)
R/rudf.R	37.00% <37.00%> (ø)

... and 1 file with indirect coverage changes

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