Expose num_chains argument to samplers besides static engine

[WardBrian]

Submisison Checklist

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

Summary:

Following stan-dev/stan#3212, we can relax most of the restrictions around the num_chains argument and simplify the structure of a lot of the code.

Intended Effect:

How to Verify:

Side Effects:

Documentation:

See stan-dev/stan#3214

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)