Add a PR checklist

[emil-ronkainen]

As a developer, it would be nice to have a checklist that outlines all tasks that should be performed before submitting a PR in order to ensure that the repo is maintainable and improve on development efficiency. This checklists would include points such as

• Have you remembered to run the test_actions.sh script?
• Have you rebased the branch on the latest main?
• Have you tested the change locally?

This could be done by adding a PULL_REQUEST_TEMPLATE.md file in the .github directory. The contents of the file would be on the form

## Checklist
- [ ] check 1
- [ ] check 2
- [ ] another check