gh-679: only import numpy when it is absolutely necessary and add doc…

[connoraird]

Description

Reduced reliance on importing numpy and glass.jax and added wrapper for importing numpy to provide a helpful message to the user. Also added detailed docstrings to _array_api_utils

this was split out from PR-677

Closes: #679

Changelog entry

Changed: How we import numpy and glass.jax in glass._array_api_utils so that we no longer explicitly depend on either of them. If numpy is incorrectly imported, a helpful error message will be provided to the user. This will hopefully reduce confusion if a user suddenly requires numpy just because they called a specific function.

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)?