Nominate Erik to a member of the Coordinating Committee

[rocco8773]

This is the official nomination and vote to add Erik Everson (@rocco8773) to list of PlasmaPy Coordinating Committee Members.

What's changed:

• [x] Add Erik T. Everson to Coordinating Committee on PLEP3
• [x] Update PLEP 3 last revised date
• [ ] Add updated PLEP 3 to zenodo and get new DOI
• [ ] Update PLEP 3 DOI
• [ ] Update PLEP 3 DOI on the README.rst
• [ ] Update PLEP 3 DOI on the PLEP-0000.rst

[namurphy]

I'm going to merge this since this was approved, and follow up with another PR before doing the Zenodo thing.