Fix stan CSV parsing for save warmups

[bdeonovic]

Summary:

Currently there is bug in parsing stan CSV output files to turn into rstanfit object using read_stan_csv which results in NA value for the field save_warmup resulting in the error

r$>  rstan::read_stan_csv(cmdstan_m_robit_3pl$output_files())
Error in if (max(save_warmup) == 0L) { : 
  missing value where TRUE/FALSE needed
In addition: Warning message:
In parse_stancsv_comments(comments) : NAs introduced by coercion

Intended Effect:

How to Verify:

using package versions: cmdstanr_0.8.1 rstan_2.32.6

Side Effects:

Documentation:

Reviewer Suggestions:

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):

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: GPLv3 (https://opensource.org/licenses/GPL-3.0)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bdeonovic]

bump?