Replace ArchiveDataFrame batch methods with `get_field`

[btjanaka]

Description

We seek to make pyribs more flexible and handle various fields. Replacing the batch methods in ArchiveDataFrame with get_field is part of this effort. Now, it should be possible to retrieve any field from the ArchiveDataFrame, not just the ones previously supported by the batch methods.

This PR also rewrites iterelites() to support more fields.

TODO

• [x] Implement get_field
• [x] Rewrite iterelites
• [x] Fix tests to use get_field
• [x] Update documentation in class.rst
• [x] Replace calls to batch methods with get_field

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