investigate online counts of post-warmup divergences (or any column) during sampling

[mitzimorris]

Summary:

When CmdStan completes a run, it should report number of post-warmup divergences and number of iterations where maxtreedepth was exceeded, and it should track this effeciently, i.e., minimal memory overhead.

We want this for cmdstanpy/cmdstanr and lightweight wrappers to avoid reading in the entire outputs.

Description:

tee sample_writer in command.hpp:

Current Version:

v2.22.1