OpenMP PSTL patch

Hi friends,

I have a patch up for review on phabricator.

I am still debugging the parallel partial sort, and I haven’t implemented the parallel merge yet, but all other parts pass unit tests. This is based off of a fairly recent HEAD (~2 weeks old).

Please let me know if you see any fundamental flaws in the approach. And of course, if I have broken any coding guidelines I will be happy to fix those things.

Sorry this has taken some time, but my day job has been quite busy. I appreciate all of the assistance that has been given.