Show a warning when user provides RDump input files

[WardBrian]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

This PR shows a warning when users provide "R dump" formatted inputs to CmdStan, letting them know that this format is essentially unsupported and new features like Tuples are not being added to it.

Documentation:

The page on RDump already recommends switching to JSON

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):

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)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[rok-cesnovar]

Thanks!

[mitzimorris]

lgtm!