Automated build testing

[rg936672]

PR Type

• Build related changes
• CI related changes

Description

Closes #854.

Adds a manually-triggered Action that builds the package, and then installs and tests the built artifact across all our supported Python/OS combinations.

Currently, this does not create a GitHub release, or upload the built artifact to PyPi, or build the documentation for ReadTheDocs. Those would be good things to add to this action in future, though - the more automated the build and release process is, the better!

How Has This Been Tested?

See https://github.com/rg936672/coreax/actions/runs/11798590544 for a successful run.

Does this PR introduce a breaking change?

No.

Checklist before requesting a review

• I have made sure that my PR is not a duplicate.
• My code follows the style guidelines of this project.
• I have ensured my code is easy to understand, including docstrings and comments where necessary.
• I have performed a self-review of my code.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.
• I have updated CHANGELOG.md, if appropriate.

[rg936672]

The performance check action is failing as it doesn't have permission to post its "No significant changes to performance" comment - this is probably because this PR is from a fork of the repository rather than from within the repository. This is something we'll need to fix if we're going to accept PRs from external contributors! Tracked at #859