R package: Make it possible to disable parallelization

[mikldk]

Thanks for your contribution!

I am running an experiment where I want to parallelize in another dimension that using OpenMP in BanditPAM. Actually, it was caused by what I think is a race condition (maybe from multiple parallelizations), and I want to avoid that - an I am already parallelizing elsewhere.

I made an option to disable parallelization, although the package is compiled with OpenMP. Please see this pull request.

[height[bot]]

Link Height tasks by mentioning a task ID in the pull request title or commit messages, or description and comments with the keyword link (e.g. "Link T-123").

💡Tip: You can also use "Close T-X" to automatically close a task when the pull request is merged.

[motiwari]

Thank you for submitting a PR! @bnaras could you take a look?

[motiwari]

I don't know enough about R to ensure this is correct, but a cursory look at the diff seems reasonable. I'll merge this now and revert if there are any user bug reports. Thank you for the contribution, @mikldk !