write transformed data along with posterior sample

[bob-carpenter]

There is currently no way to recover the data created in the transformed data block. Because that can involve randomization now, it's not possible to recreate on the outside.

There's an associated issue in the stan-dev/stanc3 repo for making the necessary changes to the generated C++ code:

stan-dev/stanc3#1522

[mitzimorris]

I think that this will involve the compiler as well - transformed data happens in the constructor and there aren't any hooks to expose the names of all data and transformed data variables.

[bob-carpenter]

Thanks for pointing that out---I opened a linked issue in stanc3 and updated the issue here with a link.