[trivial] add get_max_nij to sparse pack

PR Summary

Tiny change so that in riot we can switch to using upstreamed index split

[x] Code passes cpplint
[ ] New features are documented.
[ ] Adds a test for any bugs fixed. Adds tests for new features.
[x] Code is formatted
[ ] Changes are summarized in CHANGELOG.md
[ ] Change is breaking (API, behavior, ...)
- [ ] Change is additionally added to CHANGELOG.md in the breaking section
- [ ] PR is marked as breaking
- [ ] Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
[ ] CI has been triggered on Darwin for performance regression tests.
[x] Docs build
[x] (@lanl.gov employees) Update copyright on changed files