Add process id to generated filenames

[rok-cesnovar]

Summary

Fixes #326

We could also use this just for output csv files where this is important.

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, Uni. of Ljubljana

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]

This fixes both the example from @bnicenboim as well as

library(cmdstanr)

file <- file.path(cmdstan_path(), "examples", "bernoulli", "bernoulli.stan")
data_file <- file.path(cmdstan_path(), "examples", "bernoulli", "bernoulli.data.json")
model <- cmdstan_model(file)

out_parallel <-   parallel::mclapply(1:5, FUN = function(S) {
  out <- model$sample(data = data_file,
                      chains = 1L,
                      parallel_chains = 1L,
                      seed = S,
                      thin = 1L,
                      )
  list(output_files = out$output_files())
})

print(out_parallel)

[codecov-io]

Codecov Report

Merging #328 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #328   +/-   ##
=======================================
  Coverage   89.24%   89.24%           
=======================================
  Files          12       12           
  Lines        2538     2538           
=======================================
  Hits         2265     2265           
  Misses        273      273           

Impacted Files	Coverage Δ
R/utils.R	72.06% <100.00%> (ø)

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[jgabry]

Looks good, will merge now!