availableThreads(): Report on settings controlling amount of multi-threading

[HenrikBengtsson]

Wish

Add availableThreads(), which works similarly to availableCores(), but instead reports on settings that control multi_threading_, in contrast to multi_processing_.

It should work something like:

> parallelly::availableThreads(which = "all")
system nproc omp tbb rcpp
    16    16   4   4    8  

Open questions

• What should, say, system report on? Should it be detectCores() * detectThreadsPerCores()?
• Should this also report on anything that availableCores() report on? Basically, availableCores() * detectThreadsPerCores()?
• For queries that require loading a package (e.g. RcppParallel::defaultNumThreads()), what should happen? Should it return NA_integer_ if package is not loaded? Should it attempt to load it? (probably not)
• ...

See also

• Issue #47