Add field_list property to ArrayStore

[btjanaka]

Description

The field_desc property can provide a bit too much info on the ArrayStore; we may only want to see the list of fields. This PR provides such a property.

TODO

• [x] Implement
• [x] Document
• [x] Test

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