Davis Herring

Shouldn't we have a Spack variant for HPX?

> We do have HPX support in Spack. Right, sorry. I've been reading lots of documentation lately and not much code, and we don't _document_ the `backend=hpx` variant. I confused...

> @opensdh I have addressed the comments and squashed the commits into one. The commit message now reflects (most of) the changes made overall. So you're ready to "release the...

> @opensdh I will update this PR as I'm making progress on documenting the HPX backend. I didn't realize it at the time, but I suppose your "ready for review"...

> I'd be happy to add more if you have concrete questions or suggestions for more information. Thanks. We're going to start by moving this to be an internal merge...

Revised and merged as 3d162c4.