Serialize complex numbers to json

[WardBrian]

Submission Checklist

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

Summary

This changes write_stan_json to handle complex numbers (Python or Numpy) in the way cmdstan expects

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)

[codecov-commenter]

Codecov Report

Merging #535 (5baaf65) into develop (5ed096e) will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #535      +/-   ##
===========================================
+ Coverage    79.16%   79.20%   +0.04%     
===========================================
  Files           45       45              
  Lines         9453     9480      +27     
===========================================
+ Hits          7483     7509      +26     
- Misses        1970     1971       +1     

Impacted Files	Coverage Δ
a/cmdstanpy/cmdstanpy/cmdstanpy/utils.py	75.81% <0.00%> (+0.16%)	⬆️
cmdstanpy/cmdstanpy/utils.py	72.67% <0.00%> (+0.20%)	⬆️
...runner/work/cmdstanpy/cmdstanpy/cmdstanpy/utils.py	72.67% <0.00%> (+0.20%)	⬆️
a/cmdstanpy/cmdstanpy/cmdstanpy/model.py	88.76% <0.00%> (+0.44%)	⬆️

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[WardBrian]

@mitzimorris are you able to give this PR a quick look as well?

[mitzimorris]

great!