Add from_string classmethod constructor for CmdStanModel

[vankesteren]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

This implements a basic version of instantiating a CmdStanModel from a code string. For discussion, see #757; this implements option 3 from that discussion. I've implemented the classmethod itself as well as a (basic) unit test. I've ensured code passes both flake8 and pylint.

This is a first draft, feel free to reject this pull request, to request changes, or to completely ignore 👍 I don't want to overload you as maintainers!

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Erik-Jan van Kesteren

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[mitzimorris]

I really don't want to see this added to the code base, given that we now have the most excellent stanplayground which does this in a browser, which seems to me, where you want to be doing quick experimentation.
https://github.com/flatironinstitute/stan-playground