Conformant span

[mauneyc-LANL]

PR Summary

A span implementation that (in broad terms) follows C++20. See https://en.cppreference.com/w/cpp/container/span for more information on this.

This replaces the PR provided by @BrendanKKrueger (#47) in giving a more complete span.

@jonahm-LANL @dholladay00

TODO:

• Implement Testing
• Cleaner comments

PR Checklist

• Any changes to code are appropriately documented.
• Code is formatted.
• Install test passes.
• Docs build.
• If preparing for a new release, update the version in cmake.

[mauneyc-LANL]

Basically done. Let me know if you want something more or changed. @Yurlungur @dholladay00 @BrendanKKrueger

[Yurlungur]

Thanks @mauneyc-LANL this looks really good! Some minor questions with two moderate asks:

1. Can you add a comment in the docs that we have this backport?
2. Can you add some tests that show it compiles/runs on device with Kokkos?

[Yurlungur]

Looks like last thing is just a sentence in the docs saying this exists. THen I'm happy to merge. 👍

[Yurlungur]

Looks like there's a merge conflict. Other than that, looks good to me