Support latest PyStan3 and CmdStanPy

[defjaf]

The latest version of PyStan uses a somewhat different syntax and overall model than previous and so stanmagic doesn't work with it directly, and because of the new model -- which requires data at compilation time -- it needs more than just a simple change of names. This is sufficiently non-obvious that I've posted a Pystan3 feature request to change this behaviour.

Also I understand that CmdStanPy is becoming a popular way to do much the same thing with a python front-end, albeit with a file-based interface to stan (but I haven't used it).

One simple fix might be to just allow the magic to create a string or file with no attempt at compilation (but with syntax highlighting!)

(I tried to post a something similar in the jupyterstan repo but issues are disabled there…)

(Apologies for the lack of links in the above; the original, prettier, version of this issue was unceremoniously deleted by a github iOS bug, and I don't have the strength to replace them all right now...)