71 feature request r based user defined functions

[mattfidler]

@john-harrold as requested.

This calls a R function inside of rxode2; Still needs

• [x] function query on solve to make sure the functions are still accessible and have the same number of arguments
• [x] 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[bot]]

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%.

:exclamation: 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

```diff @@ 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](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2) | Coverage Δ | | |---|---|---| | [src/codegen.c](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL2NvZGVnZW4uYw==) | `70.04% <100.00%> (+0.26%)` | :arrow_up: | | [src/etTran.cpp](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL2V0VHJhbi5jcHA=) | `72.68% <100.00%> (-0.02%)` | :arrow_down: | | [src/genModelVars.c](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL2dlbk1vZGVsVmFycy5j) | `100.00% <100.00%> (ø)` | | | [src/init.c](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL2luaXQuYw==) | `67.30% <100.00%> (+0.64%)` | :arrow_up: | | [src/tran.c](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL3RyYW4uYw==) | `87.59% <100.00%> (+0.03%)` | :arrow_up: | | [src/tran.h](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL3RyYW4uaA==) | `98.59% <ø> (ø)` | | | [R/tran.R](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-Ui90cmFuLlI=) | `82.66% <50.00%> (+0.23%)` | :arrow_up: | | [src/parseFuns.h](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL3BhcnNlRnVucy5o) | `66.66% <87.09%> (+3.39%)` | :arrow_up: | | [src/udf.cpp](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL3VkZi5jcHA=) | `47.50% <47.50%> (ø)` | | | [R/rudf.R](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-Ui9ydWRmLlI=) | `37.00% <37.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/nlmixr2/rxode2parse/pull/72/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2)

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