Add a glossary after defining terms precisely

[atravitz]

add a glossary for terms like: bundle, group, statepoint, workspace(?), aggregate.

[cbkerr]

Closing as we are refining definitions in the google doc and the docs project.

(For future reference, I don't think this is a good first issue because it requires a deep working knowledge of how the pieces of signac fit together.)

[csadorf]

@cbkerr Why would you close the issue if the work is still ongoing?

[cbkerr]

This is the link to the google docs glossary development: https://docs.google.com/document/d/1_merhcK3ohas4IloE616yL7gypMRFcaQLh2oChExC7o/edit?pli=1