Add checks to CI that Changelog has been updated and version incremented

[davidhopkinson26]

When making a PR to add a feature to the package we should be updating both CHANGELOG.rst and the version number in __version.py to reflect the changes.

If this is forgotten the we need to make an additional PR to add these changes in before publishing a release, which slows down the release process or if forgotten means we create a release with incorrect documentation (Changelog) or cause the deployment to fail (version update).

Adding a check to the build pipeline that these two files have been updated should prevent this mistake. Having this check in place could also potentially facilitate a move to continuous deployment where a successful PR into main could automatically trigger a release workflow, should we wish to do so.

The following github action may be useful verify-changed-files