Fix: multichain does not use any init file besides the first

[WardBrian]

Submisison Checklist

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

Summary:

Currently, if you are using num_chains with initialization files for each chain, only the first is actually used. This was found by investigating a (similar, but unique) bug in cmdstanpy: stan-dev/cmdstanpy#687

The bug ultimately comes down to one character, files_1 vs files_i, and has existed since the original implementation in #987

Intended Effect:

Allow multiple chains to use separate init files

How to Verify:

New test added

Side Effects:

This also unifies the behavior of init files with the outputs etc, such that the chain ID is used rather than hardcoding numbers started at 1.

Documentation:

Copyright and Licensing

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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/)

[mitzimorris]

good catch!