feature request: Also allow number of cores to be set by `getOption("Ncpus")`

[trevorld]

• CRAN sometimes dings packages if they use more than getOption("Ncpus") cores in an example (and CRAN sets that option to what they consider a "reasonable" value for CRAN check testing). This is documented for example in ?tools::check_packages_in_dir. For example they recently dinged {piecepackr} for naively using rayrender::render_scene() in an example because I didn't explicitly turn set parallel=FALSE or manually restrict the number of cores in my example with something like options(cores = getOption("Ncpus")). If {rayrender} also allowed number of cores to be set by getOption("Ncpus") then I think future package developers using render_scene() in an example wouldn't need to manually think about throttling the number of cores in their example.

• In particular currently {rayrender} uses the following logic to set number of cores:

  if (!is.null(options("cores")[[1]])) {
      numbercores = options("cores")[[1]]
  }
  else {
      numbercores = parallel::detectCores()
  }
  if (!parallel) {
      numbercores = 1
  }

  And I'm proposing instead (for ease of reading I'll perhaps incorrectly assume you defined or imported the %||% operator) the following variant logic which I think would keep CRAN happy without developers using render_scene() need to worry about cores in examples:

  if (parallel) {
  numbercores = getOption("cores") %||% 
                getOption("Ncpus") %||% 
                parallel::detectCores()
  } else {
  numbercores = 1 
  }

[trevorld]

I can do a PR if this feature is okay.

[tylermorganwall]

This option has been added in the latest commit. Thanks for the suggestion!