Rebase onto v0.20.2

[xylar]

@ikalash, there was a new release of spack today:
https://github.com/spack/spack/releases/tag/v0.20.2
It looks like we will have an easy time with the rebase again, as you can see in my test branch here:
https://github.com/xylar/spack/tree/rebase-onto-0.20.2

I'd like to force-push the rebase but we should decide first if we want #22 to go in before or after that. I'm fine either way because I don't actually use develop directly in my own workflows.

[xylar]

@ikalash, I just remade my branch by rebasing the latest develop on to the v0.20.2 tag. Could you give it a try? If you approve, I'll force-push to develop.

[ikalash]

I tested this yesterday. I think it is OK to merge. There were failures in my nightly spack build but that was due to changes in Trilinos, and they are showing up in non-spack builds too. An issue has been opened.

[xylar]

Thanks! I just pushed my rebase branch to develop.