Fix behaviour of Masker averaging along axes

[lauramurgatroyd]

Describe your changes

Modifies Masker so that the mean and median are calculated correctly along the axis, if specified. This is a quick fix to #1530 to fix the functionality of Masker so that it is as described in the documentation. There are fundamental issues with the structure and efficiency of Masker, some of which are described in #1937. However that is beyond the scope of this PR.

Describe any testing you have performed

Please add any demo scripts to CIL-Demos/misc/ I fixed the unit tests for Masker, and also added unit tests for Masker in 3D (previously it only tested on 2D data).

Link relevant issues

Closes #1530

Checklist when you are ready to request a review

• [x] I have performed a self-review of my code
• [x] I have added docstrings in line with the guidance in the developer guide
• [x] I have implemented unit tests that cover any new or modified functionality
• [x] CHANGELOG.md has been updated with any functionality change
• [ ] Request review from all relevant developers
• [x] Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• [x] The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
• [x] I confirm that the contribution does not violate any intellectual property rights of third parties