## Summary

<!--  For example...
The existing implementation has poor numerical properties for
large arguments, so use the McGillicutty algorithm to improve
accuracy above 1e10.

The algorithm is described at https://wikipedia.org/wiki/McGillicutty_Algorithm
-->

## Related issues

<!-- For example...
Fixes #159
-->

## User Explanation

<!-- How would you explain this change to the user? If this
change doesn't create any user-facing changes, you can leave
this blank. If filled out, add the `docs` label -->

## Checklist

- [ ] reference any related issues
- [ ] updated docs
- [ ] updated unit tests
- [ ] updated UPGRADING.md
- [ ] add appropriate tag (`improvement` / `bug` / etc)
- [ ] ready for review