Veselin Dobrev

> Should we update the minimal Ginkgo version in [INSTALL](https://github.com/mfem/mfem/blob/master/INSTALL#L712) to 1.9 (it is currently 1.4)? I think we can still use Ginkgo v1.4 - there's nothing in this PR...

Some of the nightly tests failed when trying to install the library with this branch, so I added GitHub CI tests to check that the install targets work.

Re-merged in `next` for testing...

@kmittal2, I was building this PR with GSLIB 1.0.9 enabled and compiler flags like the ones we use for catching warnings in CI: `-Werror -pedantic -Wall -Wshadow` (on Mac or...

> I think I cleaned them. There are some more errors (from warnings) when you build everything with `make all` -- these are not new but it's good to fix...

Hi @adrienbernede, I created a PR (#4684) toward this PR with a set of proposed changes. It uses the changes proposed here: https://github.com/mfem/mfem-uberenv/pull/3. Let me know what you think.

@adrienbernede, I made some changes here to transition the tests from Lassen to Matrix, and from Ruby to Dane. Please take a look at the changes when you get a...

@tzanio, I just pushed a small update to increase the timeout on Matrix -- please re-merge it in `next` when you get a chance.

This seems like going a bit overboard with the palettes. 😁 The size of `palettes.cpp` is essentially doubled in terms of lines!

We primarily use and test with Apple clang on Mac, so I'd recommend using that. I have not used Homebrew clang, so I'm not sure how easy it is to...