Replace Elite and EliteBatch with dicts

[btjanaka]

Description

This PR removes the Elite and EliteBatch namedtuples from the public API; instead, we create an Elite and EliteBatch namedtuple on the fly in each archive. This allows us to support custom field names in each namedtuple in the future.

In creating this PR, I was considering whether to create a custom namedtuple for each archive (when the archive is constructed, similar to how pandas has itertuples), or to use a dict. These were the pros and cons I came up with of dicts over such namedtuples:

Pros:

• This is clearly backwards-incompatible, so users will know something has broken. The old tuple unpacking behavior will definitely not work here, and calling the attributes also will not work.
• Dicts are less finicky than namedtuples, in that there are no attributes to manage.
• Dicts are already a common data structure; people already know how to get the keys etc
• It is easier to handle retrieving just a couple of fields. In such a case, we can just add the required keys to the dict. In contrast, we would have to set some fields to None in a namedtuple
• We no longer will have a name conflict with the index method of namedtuples

Cons:

• The old unpacking logic will no longer work
• Getting attributes will no longer work
• Harder to tell which things are batch because it’s not in the name, although I think it’s usually clear from the context

TODO

• Replace all usages
• Double check for usage of Elite and EliteBatch

Questions

Status

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