export stan::math::ode_rk45 to R

[nehill197]

• Using StanHeaders package function stanFunction()
• Requires writing air2water differential equation in C++ code (using Rccp)
• e.g. https://www.jchau.org/2022/01/24/automatic-differentiation-in-r-with-stan-math/#fnref6
• e.g. https://cran.r-project.org/web/packages/StanHeaders/vignettes/stanmath.html

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[joethorley]

I'm wondering if we should do this in a new package called extrasstan to avoid having the Stan dependency in the extras package?