<!-- (Lines like this are comments and will be invisible - you do not need to edit/remove them) -->

<!-- Thank you very much for spending time on contributing to GPflow!
This template exists to simplify communicating basic information that is required to understand your contribution.
Please fill it in as far as possible; if anything about this template is unclear, please do mention it! -->

**PR type:** bugfix / enhancement / new feature / doc improvement

**Related issue(s)/PRs:** <!-- GitHub issue number, e.g. #1216 -->

## Summary

**Proposed changes**
<!-- Large PRs should ideally be preceded by a design discussion on a separate issue! -->

<!-- A clear and concise description of the contents of this pull request. -->
* ...
* ...
* ...

**What alternatives have you considered?**
<!-- A clear and concise description of any alternative solutions or features you've considered. -->

### Minimal working example

<!-- Short code snippet with relevant comments.
* Bug fixes: show what happens before (without this PR) and after.
* New feature: show different use cases and demonstrate its benefits.
-->

```python
# Put your example code in here
```

## PR checklist
<!-- tick off [X] as applicable -->
- [ ] New features: code is well-documented
  - [ ] detailed docstrings (API documentation)
  - [ ] notebook examples (usage demonstration)
- [ ] The bug case / new feature is covered by unit tests
- [ ] Code has type annotations
- [ ] I ran the black formatter (`make format`)
- [ ] I locally tested that the tests pass (`make check-all`)

### Release notes

<!-- leave blank if unsure -->

**Fully backwards compatible:** yes / no

**If not, why is it worth breaking backwards compatibility:**
<!-- include a short justification -->

**Commit message (for release notes):**

* ...