DOC: Mirror docs improvements from RStan
DEV: Update C++ wrapping for Stan 2.1.0
Squashed 'pystan/stan/' changes from 0dc2d86..cb8a910


git-subtree-dir: pystan/stan
git-subtree-split: cb8a91067d40ed64793aa88ad3ffefadbe665ce2
Update Stan source code to version 2.1.0

Merge commit 'd9c77a4fd5db11d4a6b5ec1a1c034ba5d401d8f5' into feature/v2.1.0.0