Updated version of the external Stan C++

[WardBrian]

Hi Dan -

I really appreciated the applied example of the exoplanet to showcase how and why someone might want to plug external C++ into Stan.
When adding better support for this feature in cmdstanpy I adapted the code in the blog, both to switch over from pystan and to update the C++ used to calculate the derivatives to work with more recent versions of Stan's math library.

The code is here: https://github.com/WardBrian/notebooks/blob/main/DFM-Exoplanets.ipynb

I'm not sure if you want to do anything with it, but at the very least I thought someone looking for an updated version might look at these issues

[dfm]

@WardBrian — Thanks for sharing! That link doesn't work for me. Is the repo private?

[WardBrian]

Apologies, I thought I had made it public. Should be good now.