Serialize inf/nan to json

[WardBrian]

Submission Checklist

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

Summary

This changes write_stan_json to turn Nan into "NaN", inf to "+inf", and -inf to "-inf" as CmdStan expects. It only calls this as necessary, so it shouldn't be too much less efficient than the prior check which threw an error in these cases.

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 #488 (86e7a81) into develop (725ad89) will decrease coverage by 0.37%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #488      +/-   ##
===========================================
- Coverage    77.95%   77.58%   -0.38%     
===========================================
  Files           30       20      -10     
  Lines         8991     6018    -2973     
===========================================
- Hits          7009     4669    -2340     
+ Misses        1982     1349     -633     

Impacted Files	Coverage Δ
a/cmdstanpy/cmdstanpy/cmdstanpy/install_cmdstan.py
...tanpy/cmdstanpy/cmdstanpy/install_cxx_toolchain.py
a/cmdstanpy/cmdstanpy/cmdstanpy/_version.py
a/cmdstanpy/cmdstanpy/cmdstanpy/compiler_opts.py
a/cmdstanpy/cmdstanpy/cmdstanpy/progress.py
a/cmdstanpy/cmdstanpy/cmdstanpy/__init__.py
a/cmdstanpy/cmdstanpy/cmdstanpy/stanfit.py
a/cmdstanpy/cmdstanpy/cmdstanpy/cmdstan_args.py
a/cmdstanpy/cmdstanpy/cmdstanpy/utils.py
a/cmdstanpy/cmdstanpy/cmdstanpy/model.py
... and 2 more

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[mitzimorris]

lgtm!

[ahartikainen]

LGTM