Remove metadata in favor of custom fields

[btjanaka]

Description

We plan to add the ability to specify custom fields in pyribs archives. This PR thus removes metadata, which are a current substitute for custom fields. Custom fields will also support object dtypes, meaning metadata can still be recovered by specifying a metadata field with object dtype.

While this change is backwards-incompatible, I believe it is beneficial for the simplicity of the library. Currently, metadata is the only array that is an object array, making it more difficult to save next to arrays like the objective and measures arrays. Furthermore, metadata currently has a complex defaulting system since it is usually None.

TODO

• Remove metadata from codebase
• Remove metadata from tests
• Fix docstrings

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