Support single fields in ArrayStore.retrieve

[btjanaka]

Description

This adds a shortcut whereby a single array can be retrieved from an ArrayStore. For instance, occupied, objective = store.retrieve("objective"). This call also extends to the data method, e.g., objective = store.retrieve("objective").

TODO

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go