Allow the 'vars' argument to draws_pd to filter new columns

[WardBrian]

Submission Checklist

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

Summary

Closes #717. This makes it so the columns added in #677 (e.g. chain__) can be filtered by using the vars argument, the same way e.g. lp__ can.

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): Simons Foundation

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)

[WardBrian]

@mitzimorris mind reviewing?