stansummary hangs on very large input files

[mitzimorris]

Summary:

investigate behavoir of stansummary - code defensively for CmdStanPy -

Description:

mentioned in CmdStanPy issue #88 - https://github.com/stan-dev/cmdstanpy/issues/88#issuecomment-543923076

Reproducible Steps:

Please report steps to reproduce the issue. If it's not possible to reproduce, please include a description of how you discovered the issue.

If you have a reproducible example, please include it.

Current Output:

The current output. Knowing what is the current behavior is useful.

Expected Output:

Describe what you expect the output to be. Knowing the correct behavior is also very useful.

Additional Information:

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Current Version:

v2.21.0