Open Source License

[Eric Harper]

I am no expert at open source licenses, but it look like by using Intel TBB, we are required to license freud under GPLv2.

https://www.threadingbuildingblocks.org/licensing

I had thought we might be able to use v3 (which is probably a better idea given that it's newer, but honestly GPL is quite confusing to me), but since Intel didn't include "or later" I think we have to use v2.

[Eric Harper]

As @mspells pointed out, Freud should fall under the runtime exception, so TBB iteself does not cause Freud to use GPLv2. Right now I am leaning toward the MIT license, but I would appreciate any/all input.

[Eric Harper]

Josh will used the license used by HOOMD