Add joint MMD

[dominicjbroadbent]

PR Type

• Feature
• Tests

Description

Added JMMD, fulfilling #1109

How Has This Been Tested?

Adding tests according to format of test_metrics.py.

New tests introduced with this change verify that...

Does this PR introduce a breaking change?

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.