Add DgElementArrayMember

[nilsdeppe]

Proposed changes

Upgrade instructions

Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments

[nilsdeppe]

1. added nolint for one. I don't know what to do about the other. The code seems to compile with clang so I don't know why clang-tidy is having issues.
2. Probably in the long run. I didn't mostly because we'll also need to test switching to a group instead of nodegroup to get it working with Kokkos, so just a lot of imminent changes that I don't know what they will require.
3. I've rebased and squashed. Thanks for the review!