Add the ability to specify a negative index when loading from a db

[keckler]

Description

Add the ability to specify a negative index when loading from a DB. This allows one to load the EOC node without knowing how many nodes are in that cycle.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.

[john-science]

Is this meant for this ticket?

[keckler]

Nope, different thing!

[john-science]

Sure, it's a small change. It's clear, and obvious to understand.

It doesn't break backwards compatibility.

And it's alone in a PR. Cool.