Allow multiple dicts in inits, fix multichain

[WardBrian]

Submission Checklist

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

Summary

This closes #687 and #362, and cleans up a lot of the temp file code we currently have.

The actual test is guarded behind a version test due to this cmdstan bug, but locally I have confirmed the fix works

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)

[mitzimorris]

LGTM!

[codecov-commenter]

Codecov Report

Merging #688 (e27e387) into develop (7cf6483) will increase coverage by 0.25%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #688      +/-   ##
===========================================
+ Coverage    80.31%   80.56%   +0.25%     
===========================================
  Files           72       72              
  Lines        11265    11292      +27     
===========================================
+ Hits          9047     9097      +50     
+ Misses        2218     2195      -23     

see 16 files with indirect coverage changes

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