Add output_basename

[rok-cesnovar]

Summary

Fixes #438 by adding an argument output_basename.

> fit <- mod$sample(data = data_file, chains = 4, output_basename = "custom")
> print(fit$output_files())
[1] "/tmp/RtmpHcV04N/custom-1.csv" "/tmp/RtmpHcV04N/custom-2.csv" "/tmp/RtmpHcV04N/custom-3.csv" "/tmp/RtmpHcV04N/custom-4.csv"

Combine that with output_dir to have completely predictable output CSV filepaths.

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):
Rok Češnovar

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)

[rok-cesnovar]

@wpetry @seth127 mind looking at this, if this works for you?

remotes::install_github("stan-dev/cmdstanr@output_basename"

to try it out.

[jgabry]

This looks good, I just made one comment on the doc

[wpetry]

Looks good to me too. Thanks for implementing this feature!