Cleanup output from cmdstan/bin/stansummary

[mitzimorris]

Submission Checklist

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

Summary

per issue #600 - cleanup format and contents of the pd.DataFrame that contains the output from
CmdStan's bin/stansummary report.

• don't need index header "name"
• if model doesn't contain any parameters, lp__ is full of NaNs and not useful; omit from output.

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): Columbia University

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 #602 (93aec5f) into develop (26374f8) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #602      +/-   ##
===========================================
- Coverage    80.33%   80.31%   -0.02%     
===========================================
  Files           69       69              
  Lines        10230    10233       +3     
===========================================
+ Hits          8218     8219       +1     
- Misses        2012     2014       +2     

Impacted Files	Coverage Δ
a/cmdstanpy/cmdstanpy/cmdstanpy/model.py	89.24% <0.00%> (-0.41%)	⬇️
cmdstanpy/cmdstanpy/stanfit/mcmc.py	96.63% <0.00%> (+0.01%)	⬆️
a/cmdstanpy/cmdstanpy/cmdstanpy/stanfit/mcmc.py	96.63% <0.00%> (+0.01%)	⬆️
...work/cmdstanpy/cmdstanpy/cmdstanpy/stanfit/mcmc.py	96.63% <0.00%> (+0.01%)	⬆️

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