Add rootDir arg to modify CLI

[opotowsky]

Description

I was frustrated having to change directories to get this to find the files I wanted. So I just added a little flexibility so users can specify their preferred directory to search in if needed.

This is a tiny enough add that I excluded it from the release notes -- lemme know if you prefer me to add it, though!

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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.