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]

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[motiwari]

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