PyStan 3 end-of-summer 2019 plans

I’m planning on squeezing in a bit more work on PyStan 3 before the end of summer. The main task is to get support for initial values into httpstan and, by extension, into pystan 3 (aka pystan-next).

One piece of good news: It appears that we are able to support Windows with PyStan 3 much more fully than we were able to in PyStan 2. Specifically, we can do parallel sampling in threads on Windows (thanks to @wds15 's hard work making Stan Math threadsafe).

One last note for anyone using httpstan or pystan-next on macOS or Linux. Due to what appears to be a C++ problem, the forthcoming (Stan 2.20-based) versions of httpstan and pystan-next will not work on macOS at all and will only work with the clang compiler on Linux. Everything works fine on Windows (mirabile dictu).