gh-637: Use xpx.at in fields.cls2cov

[connoraird]

Description

Adds support for jax in glass.fields.cls2cov through the use of array_api_extra.at

Closes: #637

Changelog entry

Added: jax support for glass.fields.cls2cov

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[paddyroddy]

Not if this is meant to be ready but it is failing (properly)