DEV: Update C++ wrapping for Stan 2.1.0
Squashed 'pystan/stan/' changes from 0dc2d86..fad1ae2


git-subtree-dir: pystan/stan
git-subtree-split: fad1ae230db500dc214bf070357ce838225a9ea5
Merge commit 'd7e3eae2beecac249c791c6278e02abc5d96ed93' into develop